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
- [x] 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
The block argument should be on the same line as the `do`.