The Mongo DataStore needs to pass the index options through to the Mongo driver

This change allows the `Curator::Mongo::DataStore`'s `#save` method to accept an options hash of index options to pass to the `Mongo::Collection`'s `#ensure_index` method. This will allow Curator to support all types of Mongo indexes, such as [unique indexes](http://docs.mongodb.org/manual/core/indexes/#unique-indexes), not just [secondary indexes](http://docs.mongodb.org/manual/core/indexes/#secondary-indexes).

Unfortunately I'm unsure what the best way would be for the `Curator::Repository` to pass the extra info through to the `Curator::Mongo::DataStore` without impacting the `Curator::Riak::DataStore`, so I haven't made any changes to `Curator::Repository` yet.