add gsets support

[russelldb]

This is the same as branch rdb/bet365/gsets which was already merged to basho/riak_pb develop after full review and testing. The difference here is that it is pointed at develop-2.2, and it has been rebased to bring it down to one commit

basho/riak_kv#1665
basho/riak_test#1302

[ghost]

Merges cleanly into develop-2.2 and tests pass.
On develop-2.2.5 produces the following error:

⚛︎□▷  make ; make test
==> riak_pb (get-deps)
Pulling protobuffs from {git,"https://github.com/basho/erlang_protobuffs.git",
                             {branch,"develop-2.2.5"}}
Cloning into 'protobuffs'...
error: pathspec 'origin/develop-2.2.5' did not match any file(s) known to git.
ERROR: sh(git checkout -q origin/develop-2.2.5)
failed with return code 1 and the following output:
error: pathspec 'origin/develop-2.2.5' did not match any file(s) known to git.

ERROR: 'get-deps' failed while processing /Users/bryanhunt/riak_pb: rebar_abort
make: *** [erl_deps] Error 1
WARN:  Directory expected to be an app dir, but no app file found 
in ebin/ or src/:
/Users/bryanhunt/riak_pb/deps/protobuffs
==> riak_pb (pre_compile)
==> riak_pb (compile)
WARN:  Directory expected to be an app dir, but no app file found 
in ebin/ or src/:
/Users/bryanhunt/riak_pb/deps/protobuffs
Dependency not available: protobuffs-0.9.* ({git,
                                             "https://github.com/basho/erlang_protobuffs.git",
                                             {branch,"develop-2.2.5"}})
Dependency not available: hamcrest-0.3.* ({git,
                                           "https://github.com/basho/hamcrest-erlang.git",
                                           {tag,"0.3.0-basho"}})
ERROR: compile failed while processing /Users/bryanhunt/riak_pb: rebar_abort
make: *** [erl_compile] Error 1

Shouldn't be an issue though, we're going for develop-2.2, right ?

[ghost]

@martincox - one for merging.