Math: added convenience MatrixNxN aliases for RectangularMatrix.

They were already in Magnum namespace for floats and doubles, now they
can be used also for generic type (e.g. use `Math::Matrix2x3<T>` instead
of overly verbose `Math::RectangularMatrix<2, 3, T>`). GCC 4.7+ only.
Added convenience Matrix2x2, Matrix3x3 and Matrix4x4 aliases.

Unlike Matrix3/Matrix4 these don't have any transformation-related
functions. Deprecated the old Matrix2 typedef, which is now replaced
with Matrix2x2 and will be removed in future release.
Audio: work around issue with different typedef for ALbyte on OSX.

In OpenAL-Soft (Linux, Windows) `ALbyte` is defined as `signed char`,
while it is defined as `char` on OSX. Don't know why.
Platform: missing `const` in getter.

I drank too much javascript lately.