Add support for index queries that return values

Part of https://github.com/basho/platform_tasks/issues/34

Depends on https://github.com/basho/riak_pb/pull/34 and https://github.com/basho/riak_kv/pull/502

Refactor get_index/N to specific range and eq functions.
Add timeout arguments and return_terms arguments as options to
a proplist.