Minimalist TD, closes #25

This pull request adds @gama11's Minimalist TD to the demos, per issue #25. Changes can be seen in the readme but were focused on implementing BlendMode.INVERSE for previously grey items (to maintain the black/white balance of the game), adding the ability to sell towers (per @gama11's todo), and providing cross-platform compatibility.

This demo has been tested in Flash, HTML5, and Windows. In the latter two, BlendMode effects have been replaced with a slight transparency until better implementation is available. Using BlendMode effects in either resulted in a significant performance hit. The HTML5 version suffers from blurring, as expected, and runs a bit slower, but the game itself functions fairly well.

I've also added comments were I felt they were appropriate, cleaned up and streamlined the code, and used FlxButton instead of FlxButtonPlus.

Please let me know if there are any further improvements I should make. I had hoped to get BlendModes working for native targets, but I was unable, and I would like to wrap up this project for now.

Thanks!