Add support for KV datatypes (counters, sets, maps)

* Extracts top-level types into fields as described in `yz_dt_extractor`.
* Defines content types that match those used in [`riak_kv`](https://github.com/basho/riak_kv/blob/develop/include/riak_kv_types.hrl)
* Adds fields and dynamic fields to the default schema.

TODO:

- [x] riak_test for indexing CRDTs