[Pull Request] Add ability to extend DB by driver name as well as by database name

Right now, the if you want to add a new db driver to Laravel, you have to call ```DB::extend``` for each named database.  I'd rather call ```DB::extend('odbc')``` once and be able to then use ```'driver'=>'odbc'``` in my config file.  (In my case, I have a named database that has different configs (and drivers) on dev, test, and prod.  Registering the extension by name is complicated for me, but adding drivers is easy.)