Abstracting Storage API

- Abstract Storage API
- Create WPDB adapter to handle the existing WPDB operations, should not be breaking existing code.
- Create MongoDB adapter to use a NoSQL approach, and to further abstract the APIs

- This flattens the context table into three additional fields with the main document.

WIP, the following remains:
 - [ ] Parsing Meta Query
 - [ ] Parsing Contexts Query
 - [ ] Abstracting the install/update process
 - [ ] Provide a Filter / Settings Screen to add MongoDB connection details
 - [ ] Add proper install/update routine to MongoDB, ex: Ensuring proper indexes are created

And of course, more and more testing everywhere.