Added support for Laravel Migrations

Doctrine automatically updates the database schema to represent the entities in your application, thus rendering most of the use-cases for Laravel's migrations useless, however Laravel's migrations can still be useful for migrating other stuff, such as data stored on the filesystem.

#### What's Changed

This pull request adds support for Laravel migrations, using Doctrine for the DB interaction. It adds a new Entity: `Migration`, that is automatically included into the `metadata` config. It also adds a new implementation of `MigrationRepositoryInterface` that uses Doctrine instead of Laravel's DB.

#### Caveats:

- Switching connections doesn't work (perhaps you might know how to fix it?)
- The `Migration` entity cannot be removed (perhaps I should add a config option)

It'd be great to know what you think about this, and if it is a good fit for your package.