ENH: weave: replace md5 with SHA-256

I've run testsuite locally, these were the results:

Python 3.3:

    NumPy version 1.7.1
    SciPy version 0.14.0.devced994c
    Python version 3.3.2 (default, Aug 23 2013, 19:00:04) [GCC 4.8.1 20130603 (Red Hat 4.8.1-1)]
    Ran 11184 tests in 524.427s
    OK (KNOWNFAIL=152, SKIP=540)

Python 2.7:

    NumPy version 1.7.1
    SciPy version 0.14.0.devced994c
    Python version 2.7.5 (default, Aug 22 2013, 09:31:58) [GCC 4.8.1 20130603 (Red Hat 4.8.1-1)]
    Ran 10725 tests in 441.045s
    OK (KNOWNFAIL=150, SKIP=547)

I'm assuming that this will not make its way to upcoming scipy release, although it would be really great.