[5.0] Use Doctrine Inflector

We actually already require people to download this package with laravel anyway because it is a dependency of the doctrine annotations package, so we might as well start using it. This can only be an advantage as far as I can see. It's a really small package, and takes the weight of maintaining the pluraliser off laravel.

~~NB, we used to have a second parameter in the pluralise function in laravel - I checked, and the laravel core doesn't use that parameter at all. As Laravel 5.0 is breaking, I don't see an issue with removing that param.~~ I've restored this functionality.
I think this tests should be kept and pass, excepted the different words as said by @crynobone in https://github.com/laravel/framework/pull/6762#issuecomment-67776061