SDK cleanup for easier "from-scratch" usage.

Changes in Source/ThirdParty to allow "#include <SomeLib/...>" from sources and user-code:
1) Moved "AngelScript/include/angelscript.h" to "AngelScript/include/AngelScript/angelscript.h".
2) Box2D looked clean and useable both from Source and from SDK the same way.
3) Moved "Bullet/src/*" to "Bullet/src/Bullet".
4) Moved "kNet/include" to "kNet/include/kNet".
5) Moved "SDL/include" to "SDL/include/SDL".
X) Should do Direct3D9 and OpenGL... Where to find them?

CMake files changed accordingly:
1) AngelScript:
- In AngelScript/CMakeLists.txt, the install directive copies things from AngelScript/include/AngelScript since it moved.
- From the Urho3D source files, replaced any "#include <angelscript.h>" with it's new counterpart (<AngelScript/angelscript.h>).
- Other various files changed:
  + "AngelScript/source/as_config.h": Inclusion fix (From "../include/angelscript.h" to "AngelScript/angelscript.h").
FINAL NOTES: Zero changes done in the library code itself.

2) Box2D:
FINAL NOTES: Clean and SDK-compliant, nothing to be done here.

3) Bullet:
- In Bullet/CMakeLists.txt, replaced any occurence to "src" to match the new "src/Bullet".
- From the Urho3D source files, changed any bullet include reference to it's proper new counterpart (With the leading "Bullet/" prefix).
FINAL NOTES: Many of the includes in Bullet were wrong relative to the file they were called.

4) kNet:
- In kNet/CMakeLists.txt, replaced any occurence to "include" to match the new "include/kNet", and added an include directive for it to build properly.
- From the Urho3D source files, changed any kNet include reference to it's proper counterpart. Especially, replaced all individual kNet/* include by the global kNet.h or kNetFwd.h when required.
FINAL NOTES: Most of the kNet includes were wrong... Very bad consistency from it's maintainer regarding relative paths. Most of them were re-written in the refactoring process, and since i've been told it was already a changed library, i hope it doesn't matters too much.

5) SDL:
- In SDL/CMakeLists.txt, changed the install directive to match the new SDL headers path.
- Added an include directive for SDL itself to be able to compile.
- Also changed to match the new scheme:
  + "SDL/src/events/SDL_touch_c.h": Inclusion fix.
- From the Urho3D source files, changed any reference to include SDL to match the new correct compatible scheme.
FINAL NOTES: Just one file from the SDL itself was corrected. Easy to maintain.