function addColumn returns array instead of Fluent

[mtveerman]

See:

https://github.com/sgpatil/oriquent/blob/930cf46bc89de4a94d63288e451a6c260be73b72/src/Sgpatil/Orientdb/Schema/Blueprint.php#L733

The function addColumn should return \Illuminate\Support\Fluent, but now it does not. This makes it impossible to do things like:

$table->string('something')->nullable();

Such code results in:

[Symfony\Component\Debug\Exception\FatalThrowableError]
Call to a member function nullable() on array

I've only just started using this package, so I need to dig a bit deeper before I can make a fork and post a pull request.