-Fix ScissorStack bug. This occurs when zooming far out, such that the
area of any scissor is 0. This breaks the test for the stage and ruins
everyone's life. Save result of push to see if we should pop.
-Add exception handling in the case of a body not being found in a
physics manifestation
-Make platforms static during wait time, otherwise they fall out of the
air if vertical
-Set platform to dynamic to ensure initially static platforms behave
correctly
-Add joint remove manifestation. Acts directly on world for now instead
of going through manifestation executor.
-Refactor SpriteBatch creation to constructor to ensure opengl context
2.0 is present
-Add shift immediate type feature again to group editor
Merge pull request #24 from mattchaney/master

-Merging #24 Tile Mode current status, WIP
-Add fix for upgrading worlds pre-tile mode. Will leave for at least a
week or so.
-Paint tiles by holding/dragging mouse
-Remove tiles with right click