[5.0] Add A Service Provider Contract

Allows service providers that do not extend `Support\ServiceProvider` but instead implement the contract, in the spirit of the new Contracts package.

- new ServiceProvider contract
- Foundation\Application typehints on the new contract
- call `$provider->register()' is now resolved using `->call()` so that ServiceProviders not extending `Support\ServiceProvider` can still use dependency injection.
Also, the [docs for the container](http://laravel.com/docs/master/container) talk about this access… it's part of the public API and so should be part of the contract.