Add documentation

Fixes #2.

Badges (taken from [guard](https://github.com/guard/guard)):

- [x] [Version](http://badge.fury.io)
- [x] [Travis CI](https://travis-ci.org)
- [x] [Gemnasium](https://gemnasium.com)
- [x] [Code Climate](https://codeclimate.com)
- [x] [Coveralls](https://coveralls.io)

Other things to do:

- [x] Add note about [semantic versioning](http://semver.org)
- [x] Fill out [gemspec](http://guides.rubygems.org/specification-reference/)
- [ ] Document code with YARD on [RubyDoc.info](http://rdoc.info)