Changed extension support to load from a string list of extensions

By loading extensions this way, it's not possible to later load
extensions using db.Exec, which improves security, and makes it much
easier to load extensions correctly. The zero value for the slice
(the empty slice) loads no extensions by default.

The extension example has been updated to use this much simpler system.

The ConnectHook field is still in SQLiteDriver in case it's needed for
other driver-wide initialization.

Updates #71 of mattn/go-sqlite3.
Merge branch 'extlist' into local