SDK Cleanup #2

Hi,

As mentioned in the previous discussion (See https://github.com/urho3d/Urho3D/pull/511), i have forked again and focused on a cleaner way to get a proper SDK. This is the first step of future work on the project, to allow clean SDK for any platform the "standard" way. Eg for OSX, generation of a .framework with self-contained libs, headers, etc, with an unique INCLUDE_PATH as it's usually the case.

I have done the misstake of rebasing and squashing all my commits before pushing... Everything is in one single commit again. I have could re-do it using incremental commits as suggested by @weitjong if you prefer, now that i exactly have in mind what needs to be changed and in what order.

See commit message on commit 7797150 for infos.