Ruby

This is a work in progress pull request for adding Ruby to the site.

Topics that need covering (in no particular order):

- [x] comments
- [x] basic maths
- [x] strings
- [x] arrays
- [x] hashes
- [x] methods
- [x] variables
- [x] boolean logic
- [x] control structures
  - if/else
  - for (each)
  - while
  - case
- [x] classes
- [x] modules
- [x] blocks
- [ ] method_missing
- [x] yield
- [ ] lambdas
- [ ] send
- [ ] exceptions

Please comment with anything you think I've missed