Implement a storage engine for leaderboards

This basic engine provides local storage to a Sqlite3 database of a
leaderboard. It implements the LeaderBoardProtocol that the LeaderBoard
requires.

It handles all the database creation, queries, and maintenence.
Add a primaryKey (for backends) and a description

The description lets the score be displayed in via NSLog or elsewhere as
needed.
actual implementation
Implement missing finder methods

The finders will get the data from the correct backend(s) all or just
one. It handles this internally and abstracts some of that away when
required.
Adds test code for the LeaderBoards

A quick test implementation as a driver to make sure the leaderboards
work. They do.

The output is available in the Log output in XCode. Enrolling, pushing a
score, and reading the top entries all seem to work.