Refactor Drawable2D's pixel-per-unit to Scene's 2D-unit-size attr.

Updating this Scene's attribute would mark all the attached Drawable2D components dirty.
Travis CI: API documentation update at 2014-03-11 08:21:59 UTC.
[ci package]
Improved zone gradient documentation.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	Source/Engine/LuaScript/pkgs/Scene/Scene.pkg
	Source/Engine/LuaScript/pkgs/Urho2D/Drawable2D.pkg