Organize Flixel into packages

Fixes https://github.com/FlixelCommunity/flixel/issues/194

Changes:

 * Organize Flixel, moving many of the classes elsewhere or into new packages as per [the spreadsheet](https://docs.google.com/spreadsheet/ccc?key=0Avkk09kHolxjdGlqV3BTNkRVWWU0VDFfa0JhcFJMRVE&usp=sharing)
 * Rename `FlxAnim` to `FlxAnimation`

The following are proposed changes, which we still need to confirm before I add them to the pull request:

 * Rename `FlxRect` to `FlxRectangle` - _**PRO:** Full name instead of abbreviation, just like the other classes in the same package. **CON:** Will not match HaxeFlixel._
 * Rename `FlxList` to `FlxLinkedList` - _**PRO:** More descriptive; `List` is a very ambiguous name. **CON:** Will not match HaxeFlixel._