Bug/schema naming

Proposed fix for CakeDC/migrations#131.

Adds an option for specifying the Schema class name on the command line.

Updates `_getSchema()` and `_updateSchema()` to use a new `_getSchemaClassName()` method, which merges existing behavior for plugins with a new default for base Cake applications.

Most importantly, `Config/Schema/schema.php` files will now be generated using `AppSchema` as the default class name instead of `APP_DIR . 'Schema'`.

Thanks to @burzum for the initial attempt at resolving this.