Fixed the `me.Font` and `me.BitmapFont` implementation by extending the `me.Renderable` class

That fixes the un-implemented `getBounds()` function (although this is
merely used since we removed the direct Rect implementation)
Optimized tile drawing !

`me.Tile` object now hold a reference to their corresponding Tileset,
which avoids checking for it each time we draw a Tile
Fixed documentation