Add support for indexes to Collections

Implement other indexes as well as the API to create and destroy them.

TODO
=====
- [x] Add support for hash indexes
- [ ] Add error messages on index constraint violations
- [ ] Add support for geo indexes
- [ ] Add support for skip list indexes
- [ ] Add support for full text indexes
- [ ] Update README.md
Please define this delegator at the top along with the others. And I'm not sure this documentation will lead to the desired result. I think you need the [`@!method` tag](http://rubydoc.info/gems/yard/file/docs/Tags.md#method).