Ticket #176: Fix some property collisions on TMX classes now that they inherit from `me.Renderable`

- Also make other TMX classes inherit from `me.Renderable`
- Fix `inViewport` property
- Change platform example to use `inViewport` to detect falling off the screen, instead of checking object position.
Fix TMXTileset cache.

- This was broken by accessing array references on the prototype chain; every object used the same array references.
- To see the bug in action, load the TMX Tile Map example, and switch to each map in order. When you get to the sewer map, everything will be broken.
Fix a doc comment.
Merge remote-tracking branch 'origin/master' into ticket-8