Provide resource_exists/2 callback as a fix for issue 336
Refactor yz_wm_search:resource_exists/2; add tests for PB and HTTP
Incorporate review feedback

- Replace spaces with underscores in bad index name.
- Close ProtoBuf PID after use.
Address review feedback on yokuzuna_essential for PR340

- Use yz_rt_search/4 instead of building a URL verify_non_existent_index.
- Change index string into binary in verify_non_existent_index call.
Merge pull request #340 from basho/bugfix/jra/issue-336

Fixes and tests for issue 336