FlxSpriteGroup: Remove motion var overrides, fixes #716, misc improvements

This removes the motion var overrides in `FlxSpriteGroup` (`velocity`, `maxVelocity`, `acceleration`, `drag`) and calls `updateMotion()` in the group itself, thus indirectly updating the position of each member via `x` and `y`. Are you okay with these changes, @crazysam? I think you added those var to `FlxSpriteGroup`.

I also added getters for `width` and `height` which loop through each group member. This is required to make autoRotation work in conjunction with `FlxPath` and can also be useful otherwise - however, this is probably expensive on larger groups.

A inlined a few one-liners for the group functions.

I also added warnings for some of the non-supported features like `loadfromSprite()` in debug mode.