Export randomkit functions from mtrand.pyd

Numba.random relies on randomkit functions being exported from the numpy.random.mtrand extension. On Windows these functions are not available in the mtrand.pyd extension. See https://github.com/numba/numba/issues/177.
This PR was tested with msvc9 and msvc compilers on Python 2.6 to 3.3, 32 and 64 bit. Not tested with mingw/gcc.