Alpha=0 in FlxSpriteGroup

Closes #1353 

I didn't really test it much, but I wanted to post this before going to sleep to get some comments. I had to add a new var to FlxSprite to store the old alpha value before you multiply it by 0. Then when "dividing by 0," it resets the alpha back to what it was before. The issue I brought up where setting the alpha to 0 twice in a row can't happen because it ignores setting children when Value already equals alpha.

Side note: how come `remove()` doesn't subtract away the x and y? The other stuff makes sense ignoring sorta, but if you moved a sprite from one group to another, it seems like its position would be off.