Tagged Urho3D V1.3.
Fix SDK destination installation path for Windows platform.

It appears CMake has automatically prepended "Urho3D" to built-in CMAKE_INSTALL_PREFIX variable, so we don't need to prefix the path one more time. Also in the commits:
- Fix documentation on CMake module search path for Windows platform.
- Unified SDK destination installation for Windows and non-Windows platforms.
- Replace global include directories to SDL and export header to locally scoped include directories.
- Enhance Urho3D.pc generation to also provide additional compiler flags for DEBUG/RELEASE/RELWITHDEBINFO as comments/references.
Moved SDL header include dir to 'Engine' scope as it is being referenced by a few Engine sub-directories and not just Graphics.
Improved resource unloading. Make it possible to unload all resource types by partial name at once. Editor unloads unnecessary resources after scene load or clear. Add weak ref check to all resource unloading to avoid shader programs being left without their parent shader.
Add int versions of Max, Min & Clamp to the AngelScript API.
Force elements in layout to left/top alignment, warn if has different alignment. Closes #34. Implement editor accelerator key texts a bit differently as the right alignment in a layout -hack can no longer be relied upon.
Fix NinjaSnowWar compile error.
Fix wrong build path in cmake_android.bat for detecting cache value.

Also enhance to suppress symlink creation warnings when they are already created and update Android toolchain for NDK r9b.
Fix MacOSX Urho3D shared library build when LuaJIT is enabled.

Also fix Urho3D.pc for MacOSX platform to also provide the framework list required for linking, i.e. below command works equally well with Urho3D SDK installed on Mac OS X platform.
c++ -o Urho3DPlayer Urho3DPlayer.cpp `pkg-config --cflags --libs Urho3D`
Fix Android "in-the-source" build on Windows platform.

Reinstate back the logic to read CMake cache value for USE_MKLINK the proper way as the previous shortcut approach does not work as expected when user invokes cmake_clean.bat but leaving build dir as it is.
Clean up Engine library build script.
Convert additional cflags comments into accessible pkg-config's vars.

The additional flags are usually trivial on *nix platforms but they are not so on Windows platform for MSVC.
Refactor to expect log level parameter value after a space.

To be consistent with the rest of the Engine parameters.
Match desktop OpenGL behavior with Direct3D: check that fullscreen window size is a valid resolution and do not enter fullscreen mode if it isn't. Closes #52.
Document the importance of URHO3D_STATIC_DEFINE when static linking.
Do not play audio in the editor when scene updates are paused. This allows making oneshot sound prefab objects without them losing their sound resource assignments.
Add new 'doc' target for generating Urho3D documentation.

make clean
make all
make doc      <-- New target
make install
Refactor to keep the Docs directory clean from temp. build files.

- Move the temp. build files for documentation building to its own build directory.
- Add new build option to include documentation build as part of the normal build (not enabled by default).
- Document the documentation building process.
Clean up the documetation.
Added "Is Playing" attribute to SoundSource. SoundSource no longer clears the sound resource when playback is finished. Closes #54.