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 pull request. This time, close to no code in libraries has been changed, except for kNet as it has been said that it was already heavily tweaked for Urho3D.

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:
Add TmpBuilds to SCM ignore list.
- Move AngelScript/include to AngelScript/include/AngelScript, and change CMake / Urho3D includes accordingly. In AngelScript source code, only as_config.h was changed.
- Move Bullet/include to Bullet/include/Bullet, and change CMake / Urho3D includes accordingly. No source file in Bullet was changed.
- Move kNet/include to kNet/include/kNet, and change CMake / Urho3D includes accordingly. Change most of the kNet source code include directives.
- Move SDL/include to SDL/include/SDL, and change CMake / Urho3D includes accordingly. A few auto-generated files (By original SDL configure script) were changed.