Clean up WAL files during reset

See issue #29 for why I made this change. In short, this patch just deletes the -wal and -shm files created by SQLite WAL mode. (Enabled by default on iOS7.)

I couldn't easily include my tests for this, because of the the way the SSDataKit tests are structured. But I did test this patch for both WAL mode and DELETE mode. Unfortunately, I was not able to test on older versions of iOS, but it should be just fine. As you can see, there isn't much too it.