Fixed SQLCipherStrategy, key was null. In-memory databases possible. Close cursors.

1. SQLCipherStrategy was not saving key locally, it was always null (SQLiteStrategy does it correctly).
2. If database name passed to SQLiteOpenHelper is null, database is created in memory. This was not possible since database name was the key identifying particular instance of the Strategy. In-memory databases may be useful (actually I needed it) -> new constructor added with new parameter - database name.
3. According to rule who opens a cursor should close the cursor. Without this exceptions are being thrown reporting not closed / finalized cursor.