Implement libc.math and test it

Implement libc.math and test it

Basic math.h constants and functions were added. Now when one wants to speedup
the following code::

    from math import sin, cos
    e = sin(5) + cos(6)

one can do::

    from libc.math cimport sin, cos
    e = sin(5) + cos(6)

Not all math.h features are wrapped (yet), but basic functions should work.

Signed-off-by: Ondrej Certik <ondrej@certik.cz>