Move some `FlxU` methods to `FlxPoint`

FlxU.rotatePoint() -> FlxPoint.rotate()
FlxU.getAngle()    -> FlxPoint.angleBetwee()
FlxU.getDistance() -> FlxPoint.distance()
Move some `FlxU` methods to `FlxMath`

`FlxMath` is a new Math class which is not dependent on any target
platform.

FlxU.abs()   -> FlxMath.abs()
FlxU.floor() -> FlxMath.floor()
FlxU.ceil()  -> FlxMath.ceil()
FlxU.round() -> FlxMath.round()
FlxU.min()   -> FlxMath.min()
FlxU.max()   -> FlxMath.max()
FlxU.bound() -> FlxMath.clamp()
Move `FlxU.getTicks()` to `FlxG.getTicks()`.

FlxU.getTicks() -> FlxG.getTicks()

Also removed obsolete ASDoc comment.
Move `FlxU.computeVelocity()` to `FlxMath`

Also includes some slight canges to the arguments (instead of
automatically retrieving the time elapsed from FlxG, users are required
to include that information as an argument.)

Also updated any references that were still using the old version.

    FlxU.computeVelocity() -> FlxMath.computeVelocity()
Merge pull request #208 from IQAndreas/reorganize-FlxU

Reorganize `FlxU`