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.

License header was not added to COLLADA test files (because it should be
then also added to TGA test files, which is impossible).
Relicensing to MIT/Expat license, part 4: CMake and documentation files.
Relicensing to MIT/Expat license, part 5: updated package files.
Relicensing to MIT/Expat license, part 6: mention it in documentation.