Updated FindCorrade.cmake from Corrade repository.
Updated Find*.cmake modules from Magnum repository.
Relicensing to MIT/Expat license, part 1: new license document.
Relicensing to MIT/Expat license, part 2: headers.
Relicensing to MIT/Expat license, part 3: source files.

Added license header also to GLSL sources, hopefully it won't hurt
compilation times too much.
Relicensing to MIT/Expat license, part 4: CMake and documentation files.
Relicensing to MIT/Expat license, part 5: mention it in documentation.