New method: SessionStore::increment

Several stores in the framework offer a method increment,
as a convenient helper method for get/set.

This change implements an increment method in the session store,
to ease the tracking of a state into a session.

For example you can:
```
        $redirectsCount = Session::increment('auth.redirects');
        if ($redirectsCount < 3) {
            // Retry to authenticate user to an external service.
        }
```

src/Illuminate/Session/Store.php

@@ -413,6 +413,22 @@ public function push($key, $value)
         $this->put($key, $array);
     }
 
+    /**
+     * Increment the value of an item in the session.
+     *
+     * @param  string  $key
+     * @param  mixed   $amount
+     * @return mixed
+     */
+    public function increment($key, $amount = 1)
+    {
+        $value = $this->get($key, 0) + $amount;
+
+        $this->put($key, $value);
+
+        return $value;
+    }
+
     /**
      * Flash a key / value pair to the session.
      *

tests/Session/SessionStoreTest.php

@@ -251,6 +251,23 @@ public function testClear()
         $this->assertFalse($session->getBag('bagged')->has('qu'));
     }
 
+    public function testIncrement()
+    {
+        $session = $this->getSession();
+
+        $session->set('foo', 5);
+        $foo = $session->increment('foo');
+        $this->assertEquals(6, $foo);
+        $this->assertEquals(6, $session->get('foo'));
+
+        $foo = $session->increment('foo', 4);
+        $this->assertEquals(10, $foo);
+        $this->assertEquals(10, $session->get('foo'));
+
+        $session->increment('bar');
+        $this->assertEquals(1, $session->get('bar'));
+    }
+
     public function testHasOldInputWithoutKey()
     {
         $session = $this->getSession();