Fix for negative object width and height

- Turned `width` and `height` of `FlxObject` into properties with warnings to
ensure they can't be set to negative values, which breaks collision
detection while still being drawn correctly in `visualDebug`, causing confusion
- Also improved the documentation of `width`, `height` and `offset` to be more
beginner-friendly and informative

Originally, I wanted to prevent setting the values to 0 as well, but turns out that that's the default value when creating a new `FlxObject`. It's probably not a big deal you can do that, but I thought it shouldn't be possible since it's basically a workaround for setting `solid = false´ and doesn't really have any other use.

This fix was inspired by [this forum thread](http://www.haxeflixel.com/forum/help/does-player-can-crouch).