Adds support for object layers

@Kibo,

Here are some updates I made to TiledMapBuilder to get support for objects layers, which I guess were added to Tiled after you wrote this library.  I thought the updates might be useful to others, so I figured I'd send a pull request.

I used your library in my Ludum Dare #30 entry.  https://github.com/periodic/LD30

It was super-useful to to be able to integrate Tiled maps into Crafty so non-technical team members could build levels without too much trouble.

The white-space in these files is a bit weird.  I tried to match yours as much as possible.