Extend brew-test-bot to support taps

This is my stab at trying to extend brew-test-bot to support taps.

I've briefly tested this with `brew test-bot --local --tap=homebrew/homebrew-php 983d939617eec6e6b9dbd39d43f7c18f2c5e619b`, which seems to work, but I could have missed something.

I'm a ruby newbie, so be gentle ;)
I know it's probably a pain but can you try testing the options below this (`--ci-*`) and setting the necessary Jenkins environment variables to build particular commits? No worries if you don't have a chance; otherwise I'll give it a go when I do.