Python reverse_http Stager

This pull request adds a reverse_http stager for Python with support for HTTP proxies. By default, if no proxy is set by the user then Python will try to determine one from the environment. The exact places it checks are dependent on the OS and some details can be found in the urllib2 and urllib.request documentation.

Stay tuned for a PR to include a reverse_https version as well.

Tested on the following versions of Python:
 - Python 2.7 on OS X
 - Python 3.4 on OS X
 - Python 2.5-2.7 & 3.1-3.4 on Ubuntu
 - Python 2.5 on Windows XP
 - Python 2.7 on Windows 7
 - Python 3.4 on Windows 7

Testing Tasks:
 - [x] Get a session over http
 - [x] Verify that the ```detach``` command works as expected
 - [x] All checks in post/test/meterpreter pass (16 pass, 0 failures)
 - [x] An unauthenticated HTTP proxy can be specified and used