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
You're re-creating the index you already defined in the `ArticlesCollection` above and then check if it was created. Why not just check if there is an hash index on the given attribute for the given collection. The API provides [such a method](http://docs.arangodb.org/HttpIndexes/WorkingWith.html#read-all-indexes-of-a-collection). And Ashikawa::Core [does this too](http://www.rubydoc.info/gems/ashikawa-core/Ashikawa/Core/Collection#indices-instance_method).