Fixing Issue #38

This will fix the issue with the ladder. According to the comments, I used the same trick by using a topladder tile and a ladder tile.
I review the collision process to ensure this is working. I also commented the code, so the documentation will be fine.

Additionnally to that fix, I add a function getEntityByProp, that allow to retrieve an object by a custom property (not only GUID as already existent).