Add missing convenience methods

[rhyshort]

Adds missing convenience methods to CDTDatabase.

CDTDatabase now supports all CRUD actions for documents, Create and Delete actions for query indexes and finding documents via query.

[mikerhodes]

If I'm honest, I'm a bit dubious about what these methods give the developer -- I'd prefer to avoid adding code if I'm not sure its needed.

[rhyshort]

I think it gives a nice single point of entry for those people who are new to Cloudant and objective-cloudant, it's basically, here is your starting point, provides basic functionality, then for full control for each operation it points them towards the backing Operation subclass where they can tweak each option to their hearts content.