Complete Javadoc rewrite, bug fixes

I have re-written most of the Javadoc to make it match the general
guidelines. I have added descriptions for every method parameter, fixed
wrong docs, improved the general markup and checked for bugs in the
methods themselves.

An issue I fixed that is worth mentioning is that layers previously
didn't receive a Display instance when update() was called, which is now
changed. You have to make appropriate changes if using LayeredLevel for
your game.

Additionally, the README file was updated. It now includes a link to an
online version of the Javadoc.