Math: doc++, minor code cleanup.
Math: matrix/vector rework, part 2: matrix as array of column vectors.

Overall architecture is simplififed with this change and also it's not
needed to use reinterpret_cast in matrix internals anymore, thus there
is no need for operator() and [][] works now always as expected without
any risk of GCC misoptimizations.

On the other side, constructing matrix from list of elements is not
possible anymore. You have to specify the elements as list of
column vectors, which might be less convenient to write, but it helps to
distinguish what is column and what is row:

    Matrix<2, int> a(1, 2,                   // before
                     3, 4);

    Matrix<2, int> a(Vector<2, int>(1, 2),   // now
                     Vector<2, int>(3, 4));

For some matrix specializations (i.e. Matrix3 and Matrix4) it is
possible to use list-initialization instead of explicit type
specification:

    Matrix<3, int>({1, 2, 3},
                   {4, 5, 6},
                   {7, 8, 9});

I didn't yet figure out how to properly implement the general
(constexpr) constructor to also take lists, so it's a bit ugly for now.

Matrix operations are now done column-wise, which should help with
future SIMD implementations, documentation is also updated accordingly.
I also removed forgotten remains of matrix/matrix operator*=(), which
can be confusing, as the multiplication is not commutative. Why it is
not present is explained in d9c900f076f2f87c7b7ba3f37a3179c0c0e4a02c.
Math: matrix/vector rework, part 3: conversion as explicit constructor.

Also updated all dependent classes to follow the change, such as Color
and Rectangle. Backwards compatibility for GCC 4.6 (with lack of support
for delegating constructors) will be done as non-constexpr constructor
using operator=().
Math: creating Vector from one value is broken.

Added "expect fail" test, it is not the priority now (and it is handled
correctly in subclasses).
Math: new class BoolVector.

Will handle output from various comparison operators in Vector.
Math: matrix/vector rework, part 4: using BoolVector for comparisons.

The comparisons have also SIMD instructions returning bool vector, this
future-proofs them. Also it isn't confusing anymore (a < b is true when
all are less or just one?).