Add ability to Delete records using a predicate.

Added a Delete function which allows records to be deleted using a predicate.

See StackOverflow question for details:
http://stackoverflow.com/questions/21123170/is-there-a-way-to-delete-in-sqlite-net-by-using-a-predicate/22055268#22055268

Can delete using:

    conn.Table<MyEntity>().Delete(t => t.someProperty == someValue);