Organize Flixel into packages

[IQAndreas]

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 created by Ka Wing
• 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.

[Dovyski]

About the renames, I think we should keep FlxRect and FlxList.

[Dovyski]

Feel free to merge this anytime, so we can start working on the remaining tasks without merge conflicts :)

[Gama11]

I went ahead and made the FlxList -> FlxLinkedList change in HaxeFlixel. It's a way better name in my opinion. :)

[Dovyski]

Awesome! :D