Lazy Load Doctrine's SchemaTool

When running `php aritsan` or any `artisan` command all commands are initialized. The modified schema related commands all require an instance of `SchemaTool`. Since `SchemaTool` requires a correctly configured database connection, if you have an incorrectly configured instance `php artisan` will crash. This effects all artisan commands and in turn effects composer commands. The below change will lazy load the `SchemaTool` preventing `php artisan` from crashing until the commands themselves are actually executed.