Truncate method added to empty the full table and reset its id

Viewing the issue #92, I've add a static method to Model to have a way to clear table contents and reset its incremental ID.

Solution is not very clean, I have use raw SQL and execSQL, but I could not find another way of doing this without modifying a lot of classes and structure (maybe add a sqlite_* TableInfo?). Anyways I hope it helps (It have been tested with suite).