removed the check on the `alwaysUpdate` property for non-renderables objects

also removed the `alwaysUpdate` property from the Tween Object
removed the `z` property from the Tween object definition

this is now enforced through the `addChild` function that will set the
property to `Infinity` if not defined.

the Tween object is now almost the default one without any specific
(melonJS) properties.
Changed the test order so that the one on `isRenderable` is the first one (`inViewport` or `visible` and not defined for non-renderables)
[#331] fixed a couple of documentation typos
Merge branch 'master' into 1.0.0-dev