Added SQLite3 driver to achieve Symfony2 compatibility

+ Add new SQLite3 driver with sqlite3:/path scheme, which uses proper PHP SQLite3 driver instead of global namespace functions. Using the existing sqlite driver raises 'Attempted to call function "sqlite_open" from the global namespace' when used with Symfony2.

+ Add create IF NOT EXISTS for generateCreateTable function in Sql.php and
all inherited drivers so that the db driver will not complain when asked to create a table while it has already existed.