[#204} fixed (bis) `adjustPosition`  when gid is equal to `0`
[#201] allow string parameters for `renderable.addAnimation()` when used with TexturePacker

- I'm not really happy with it yet, as passing string values, currently
make the code parsing the atlas for each of them, but I did not come
with a better way (except of adding one more dictionary with the
name/index correspondence)

- Also simplified a bit the implementation for "fixed-size cells"
spritesheets, and got rid of `this.spritecount` (one less useless object
property per entity!)
[#201] fixed parameter's type in documentation
[#66] small change in the default css to lower the font size