Add Maxreg, Minreg and RangeReg as new supported types

for 2.next

Goes with basho/riak_kv#59 - my work creating these simple data types.

This adds them both as top-level types, and as embedded types (within maps). The former should work very simply (though perhaps would make tiny riak_objects), the latter is untested but it's in my queue to do.

TODO:
- [ ] merge basho/riak_kv#59
- [ ] update basho/riak_pb to support these as top-level types
- [ ] test these all the way through the stack