Added capability for "Direct Piping" - that is, transferring data bet…

…ween

2 processes directly using OS pipes and connecting them with dup2, instead
of buffering the output of process 1 into python memory before sending to
process 2.

This is in response to issue amoffat#119 amoffat#119

The effect can be enabled by setting call arg _piping="direct" on the inner process.
I had issues with getting 'Input/Output error reading stdin' from dd, until
I set _tty_out=False. In the code I have therefore set _tty_out to False when
_piping == "direct". This has caused 3 of the unit tests to fail.

test2.py on my PC shows 13s vs 0.5s performance difference

test2.py

@@ -0,0 +1,28 @@
+#!/usr/bin/python
+import sh
+import time
+import sys
+
+class Timer:
+    def __enter__(self):
+        self.start = time.time()
+        return self
+
+    def __exit__(self, *args):
+        self.end = time.time()
+        print "Time taken: %s" % (self.end - self.start)
+
+def debug(line):
+    sys.stdout.write(line)
+
+print "Original piping method:"
+with Timer() as t:
+    sh.dd(sh.dd("if=/dev/zero", "bs=1M", 'count=512', _piped="out", _err=debug), "of=/dev/null")
+
+print "Direct piping method:"
+with Timer() as t:
+    sh.dd(sh.dd("if=/dev/zero", "bs=1M", 'count=512', _piped="direct", _err=debug), "of=/dev/null")
+
+
+print 'done'
+sys.exit()