ticket #15 : first pass update
backporting some change to avoid merge conflict with main branch
manually merging changes from main branch
Merge branch 'master' into ticket-15

Conflicts:
	src/entity/entity.js
further commits on ticket#15

- rename the `anim` property to `renderable`
- new `me.Renderable` class from which `me.SpriteObject` inherits
- renderable ojbect position is relative to the entity position
- fix small issue with uipdate function when flickering
renamed this variable
Removed the `me.InvisibleEntity` as it does not contain anything anymore
updated other examples
Merge pull request #165 from obiot/ticket-15

Ticket #15 - inheritance / composition