Add ruby-watchman gem

This is a RubyGem with a C extension that makes it easy to communicate fast with Watchman using its binary protocol from Ruby. We already have a `python/bser` directory that fulfills a similar purpose for Python projects.

This is code I wrote for a side project[0]. During the FB hackathon, extracted and wrapped it into a gem. All RubyGems have an explicit license, so I used the same license we're using in `python/bser`.

[0]: https://github.com/wincent/Command-T/commit/4b2837e8c1312be334e64b