Make riak compatible with R16

This PR depends on the following PRs:

- https://github.com/basho/riak_search/pull/138
- https://github.com/basho/riak_kv/pull/597
- https://github.com/basho/webmachine/pull/156
- https://github.com/basho/riak_core/pull/348
- https://github.com/basho/eper/pull/6

Note that some of the changes needed aren't well-covered by unit tests, so if you can think of some way to exercise them, please let me know.