CRDT modules need new/1, new/2 arities

[drewkerrigan]

The current CRDT modules do not support the riak-erlang-client's new/1 and new/2 arities making it difficult to "hydrate" certain types of objects.

[drewkerrigan]

Additionally, some of the Riak.CRDT.Map functions are not consistent with regard to keys ({string, atom} vs just string). These should be standardized, or at least handled with multiple function clauses.