Use Munit tests and coverage test for travis-ci

Here is a basic start to some general testing for HaxeFlixel using https://github.com/massiveinteractive/MassiveUnit. 

You can see basic examples in the ./tests folder of the engine running using @Async with a collide test on a FlxSprite.

This enables a base for more unit like tests for the core features of HaxeFlixel and can aid us in keeping things stable.

Currently I was only testing this with swf, possible to look at cpp and other targets for the future.

Will need to document this more, @Gama11 confirms its working on windows, for me its working on mac. 

To run:
- `haxelib install munit`
- it uses the haxelib versions of flixel etc
- set your `haxelib flixel dev` to this version of flixel
- open ./test and run `test.bat` or `test.sh` depending on your platform.


Travis reported the [mcover](https://github.com/massiveinteractive/MassiveCover) coverage tool is returning fine too.