Implement ensureIndexed functionality

- Implement ensureIndexed functionality as it exists in
CloudantQueryObjC.
- Added IndexManager.java, IndexCreator.java, IndexUpdater.java, and
ValueExtractor.java.
- Code is still untested and JUnit tests still need to be added.
- Remove build.gradle dependencies for Spock library since we will not
be using Spock for testing purposes.