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 ;)
Taps are not required to have a `Formula` directory (for example, `homebrew-versions` and `homebrew-science`) so this logic is incomplete. The formula discovery rules are: first check for a `HomebrewFormula` directory and use that if it exists, then `Formula`, then the repository root.