Simplify App by always using default loop, drop optional loop instance

[clue]

This changeset simplifies the App by removing the optional $loop argument and always using the default loop. Among others, this simplifies usage, improves the API documentation, removes a bunch of ambiguous tests, and overall helps clean up the project. Overall, we have yet to see a use case that requires an explicit loop instance and given that fibers require a single, shared loop instance anyway (reactphp/async#15), it's unlikely we'll find many use cases for this.

This is obviously a BC break for consumers that currently explicitly pass a loop instance, but it is believed that this should not affect most consumers of this package. If you are affected, updating this is very straightforward:

// old
$loop = React\EventLoop\Loop::get();
$app = new FrameworkX\App($loop); 

// new (already supported before)
$app = new FrameworkX\App();

Builds on top of #22, #29 and #81