Add Sqlite as a backend for notes storage

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