Handle zone search better to avoid erroneous lower-priority zone assignment. Added OnRemoveFromOctree() virtual function to Drawable. Zone code simplified. Remove zone reference from drawables within its bounding box when the zone is being destroyed.
When a Zone moves or is removed from octree, clear zone from all drawables within the bounding box, not just from those that belong to the zone. This allows zone search in View to perform more optimally.
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.
Also recalculate bounding box when Scene's 2D unit size change.
Use proper object return type in Lua bindings of UIElement::GetChild() to enable subclass properties & functions.
Fixed the Lua binding of UIElement::GetParent() to return the proper type.
Fix MINGW_ROOT variable value computation when not being provided.
Add Box2D license.
Make MarkAllDrawable2DDirty function private in Scene class.
Add Box2D also to library list in Urho3D.dox.