First iteration for the new plugin system

This is pull request 1 of 3 regarding the new plugin system. Here's what I've done so far:

- Implemented `FlxSignal` and `FlxG.signals`. There are a couple signals already working (e.g. `preUpdate`);
- Refactored `TimeManager` and `DebugPathDisplay` to use the new plugin architecture;
- Created a new interface (`FlxPlugin`) to define the structure of any plugin;

What is missing (next tasks):
- Some docs;
- Refactor the record functionality to make it a plugin.

Please give feedback about the code and the architecture. I've tried to mimic HaxeFlixel API/names/packages when possible. After this pull request gets merged, I will start working on the record plugin.