Adding SSH Agent forwarding basic support

[bendavis78]

I've ported over the pull request from paramiko that implements basic SSH agent forwarding support (see here).

It seems from previous discussions that no one really knows a good way to write official tests for this, but, I can say that it seems to work well for me in the few times that I've tried it with fabric.

[bitprophet]

Thanks for taking care of porting this.

Unfortunately, the original patch uses select.poll which is unavailble on stock OS X Python builds, and is thus unacceptable (I require that Fabric & its dependent libraries work on Apple's Python builds since a large portion of the userbase, myself included, use them.)

I'll try to take a stab myself at replacing it, a quick check implies that select.select and select.poll are roughly the same in capability (with poll being more efficient at large numbers of connections, which should not be a significant problem in most use cases) even if the API is quite different.

[bitprophet]

Easier than I expected, thankfully. And...less verbose than what it replaces. Still works fine on Linux, not that I was worried it wouldn't.

While I'm here I think I will address some other uglies, like old-style classes (seriously? jeez)

[bitprophet]

This now works OK for me on both OS X (Python 2.5) and Linux (Python 2.7). I'm going to close it out, though I may try to get some folks to beta test before it goes out for reals and will reopen if issues come up.