Add returncode property; fix iteration terminating early.

The returncode is a useful property to be able to pull out.
The iteration would terminate early if the process had already exited
by the time that the call was made. We now follow the documentation
and return the results until the buffer is empty.

streamedinput.py

@@ -250,6 +250,12 @@ class StreamedInput(object):
             raise StreamedInputNotStartedError("StreamedInput object cannot be sent data before starting")
         self.proc.stdin.write(string)
 
+    @property
+    def returncode(self):
+        if not self.proc:
+            return None
+        return self.proc.returncode
+
     def _terminate(self):
         """
         Internal call to issue a TERM signal to the subprocess.
@@ -320,7 +326,7 @@ class StreamedInput(object):
         if self.proc is None:
             raise StreamedInputNotStartedError("StreamedInput object cannot be enumerated before starting")
 
-        while not self.proc.poll() and len(self):
+        while len(self):
             item = self.read()
             if not item:
                 break