Caching expansion and test prompt

Code review for a couple new features. Initially I was planning to simply add caching for GETCONF and SETCONF requests, but ended up making a few other changes on the way.

The most prominent is a new 'prompt' script that you can use to quickly get python interpretor with a controller instance. Handy for testing.

Changes are available in my caching branch... git clone -b caching git://git.torproject.org/user/atagar/stem.git https://gitweb.torproject.org/user/atagar/stem.git/shortlog/refs/heads/caching

I'd suggest looking at the commits individually via "git log -p --reverse origin/master.." since they're each for something different, and the commit message should give a good description of what it's for.