SQLite3 for All

This PR exposes our version of @praeclarum's SQLite3 driver as public and allows you to get to the underlying SQLite3 connection from the concrete instance of `SqlitePersistentBlobCache`. This is super handy for times where you actually need a query engine on top of some data, but you don't want to create an entirely separate SQLite3 db. 

To prevent collisions with people already using a different version of the SQLite3 driver, we hide ours in a separate namespace, `Akavache.Sqlite3.Internal`

/cc @Haacked who was interested in this