ExitPolicy Class for stem

"Arm has a readily available class for representing a Tor exit policy ( in torTools.py). We need to adapt this class to stem's coding styles, write a constructor for it (see the getExitPolicy() and getRelayExitPolicy() methods for examples), and see if we can improve on it. This will also need quite a bit of testing.."

Preliminary stab at this - Just to make sure I'm going in the right direction.

https://gitweb.torproject.org/user/gsathya/stem.git/shortlog/refs/heads/exit_policy

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information