Added close_abort() function to WiFiClient

This branch adds a close_abort() method to the WiFiClient class.

How it works, what it does:
Calling `close_abort()` will close and abort the client connection it
is invoked on and, as a result, free its resources (i.e. memory).

**WARNING:** aborting connections without a good reason violates the
TCP protocol, because a closed connection would normally need to
spend some time in `TIME_WAIT` state before its resources are freed.

Usage example:
    WiFiClient client;		// set up a client

    { /* do things with your client */ }

    client.stop_abort()		// when you're done,abort the
				// connection if you must

Why it's useful:
1. Give programmers a way to shut down connections immediately if
   need be. The underlying `tcp.c` file has an abort function, but
   this has not been directly accessible via the `WiFiClient`
   class until now.

2. There are a number of reported issues for the repository
   addressing the heap corruption that can result from trying to
   retain too many connections in `TIME_WAIT` state (most notably:
   esp8266#230, esp8266#1070, esp8266#1923). Although the warning above holds, there may be
   circumstances where this isn't very important. For example an ESP8266
   running in AP mode hosting a page, which requests a new
   connection every second via an AJAX script to  monitor
   a sensor/button/etc. continusously.

Currently existing alternative approach:
When building a project, defining the
`-D MEMP_NUM_TCP_PCB_TIME_WAIT=5`compiler directive will limit the
maximum number of clients allowed to stay in TIME_WAIT state. `5` is
the default, lower it as necessary. See reference
[here](https://github.com/esp8266/Arduino/blob/master/tools/sdk/lwip/include/lwipopts.h#L263)

Thanks:
Thank you to @me-no-dev, @everslick and @Palatis for bringing the `
MEMP_NUM_TCP_PCB_TIME_WAIT` option to my attention.

libraries/ESP8266WiFi/src/WiFiClient.cpp

@@ -269,6 +269,15 @@ void WiFiClient::stop()
     _client = 0;
 }
 
+void WiFiClient::stop_abort()  //Compare this to ::stop
+{
+    if (!_client)
+        return;
+
+    _client->unref_abort();
+    _client = 0;
+}
+
 uint8_t WiFiClient::connected()
 {
     if (!_client)

libraries/ESP8266WiFi/src/WiFiClient.h

@@ -64,6 +64,7 @@ class WiFiClient : public Client, public SList<WiFiClient> {
   }
   virtual void flush();
   virtual void stop();
+  virtual void stop_abort();
   virtual uint8_t connected();
   virtual operator bool();
 

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -86,6 +86,27 @@ class ClientContext
         }
         return err;
     }
+
+    err_t close_abort()
+    {
+          /* ERR_ABRT must be sent back on abortion as specified in the comments
+           * of tools/sdk/lwip/src/core/tcp.c at the footnote of tcp_abort() */
+          err_t err = ERR_ABRT;
+          if(_pcb) {
+              DEBUGV(":close\r\n");
+              tcp_arg(_pcb, NULL);
+              tcp_sent(_pcb, NULL);
+              tcp_recv(_pcb, NULL);
+              tcp_err(_pcb, NULL);
+              tcp_close(_pcb);
+              /* Without delay some clients fail to receive the response and 
+               * report a 'cannot connect' error message */
+              delay(10);
+              tcp_abort(_pcb);
+              _pcb = 0;
+          }
+          return err;
+	}
 
     ~ClientContext()
     {
@@ -123,6 +144,21 @@ class ClientContext
             }
         }
     }
+
+    void unref_abort()
+    {
+		if(this != 0) {
+		    DEBUGV(":ur_abrt %d\r\n", _refcnt);
+		    if(--_refcnt == 0) {
+		        flush();
+		        close_abort();
+		        if(_discard_cb)
+		            _discard_cb(_discard_cb_arg, this);
+		        DEBUGV(":del\r\n");
+		        delete this;
+		    }
+		}
+	}
 
     void setNoDelay(bool nodelay)
     {