Added support for cross-compiling Urho3D for Windows on Linux systems.
Changed includes of Windows.h to windows.h so it's possible to cross-compile on Linux
Added a section about cross-compiling with MinGW in the Readme.
Copied MinGW build instructions from Readme.txt to GettingStarted.dox
Merge remote-tracking branch 'remotes/wibbe/mingw-cross-compile-toolchain'