Support for more map layers

[nelsonxb]

Some extra things that should be encoded into the map.

• Backdrop image
• Pickups/collectables
• Action tiles (e.g. signs, doors)

[nelsonxb]

I've added these three features into the map file (ea2ba7c), ready to be implemented.

[nelsonxb]

I note that Cocos doesn't parse Image type map layers. If I try to level['backdrop'], I get a KeyError (i.e. that layer "doesn't exist" as far as Cocos is concerned). Which could make this a bit tricky...

EDIT: Looks like it's not being worked on upstream yet, so we'll have to handle it ourselves. Shouldn't be too difficult though, considering we can just lift the existing parse code (yay open source!).