Add eunit for riak_dt_gcounter.
Add eunit for riak_dt_gcounter.

Add a bunch of eunit tests for the integration with the
riak_dt_gcounter.

ergl/lasp,ergl/lasp,lasp-lang/lasp,lasp-lang/lasp,lasp-lang/lasp,lasp-lang/lasp,ergl/lasp,ergl/lasp