RUBY-564 Support for (hashed, text, 2dsphere) index types

Added in constants for hashed, text, 2dsphere index types.

Added in conversion of bucket_size to bucketSize for use with geoHaystack indexes in the create_index and ensure_index methods on a collection.

Included text, hashed, 2dsphere in the list of valid "directional" index types when creating an index.

Wrote tests that ensure 2dsphere, geoHaystack, text, and hashed indexes can be created.  NOTE: the test on text index creation is commented out for now because it is an experimental feature of the server and will throw an error if the mongod isn't started up with '--setParameter textSearchEnabled=1' at present. 

Updated documentation for all affected methods.