Fix regression in audio looping

- This is actually a bug in Howler: https://github.com/goldfire/howler.js/issues/250
- Fixed, and will be committed upstream
- Also fixed the callback with looping sounds (do not automatically cleanup the callback when looping is enabled)
me.Font|me.BitmapFont: Fix documentation

- me.Font requires a Context2d reference
- me.BitmapFont requires a Renderer instance
Merge branch '2.0.x'