Add Sqlite as a backend for notes storage

[apodda]

Add a class which relies on an sqlite full text search table rather than on json files for notes storage, and a configuration option to switch between the two. The sqlite backend greatly improves startup time and search speed, especially in case insensitive mode. It also allows for boolean searches.

These features are still missing from the new backend:

• Simplenote sync
• Writing notes to file
• Sorting and pinned notes
• Regex-style search

[cpbotha]

This is really awesome, and something I really think we should do.

The most important thing we need to get working, is the simplenote sync. You've done a sterling job writing a new NotesDB for the sqlite backend. I think if we could also factor out the syncer, then we would have seamless support for the different storage backends.

I hope I get some time to look at this in the coming weekes.