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

In Mongo module: Added in constants for hashed, text, and 2dsphere index types.  Added array constant for 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 hashed, text and 2dsphere in the list of valid index types when creating an index.

Wrote tests that ensure 2dsphere, text, geoHaystack, and hashed indexes can be created.  NOTE: the test on a text index is commented out because the server must be started with 'setParameter textSearchEnabled=1'

Updated documentation for all affected methods, specifically references to index types as "directions".