Support for proxies

[moliware]

Implements the feature explained in issue #54

Now you can do a GET/POST/.... through a proxy.

E.g:

>>> requests.get('http://docs.python.org/', proxies={'http' : 'localhost:3128'})
< Response [200]>

[kennethreitz]

This is awesome, thanks! I'm going to toy around with it first before I merge. I'll get back shortly.

[kennethreitz]

🍰

[moliware]

Thanks! I was using requests ( nice job, it's really useful ) for a couple of weeks and I need this feature on a project at work.