[GH-566] Added missing return statement to AbstractCommand.

This issue is created automatically through a Github pull request on behalf of BenMorel:

Url: https://github.com/doctrine/doctrine2/pull/566

Message:

The `AbstractCommand::execute()` method should return an `integer`, or `null`.

Although in PHP, omitting the `return` statement is equivalent to returning `null`, it is recommended to put an explicit `return` to clearly differentiate it from `return void`.

I've added `return 0;` here, keeping the existent behavior.