Use internal hashlib.md5 instead of external md5sum

/pyglance/glance/data.py in FileInfo/__init__ around line 501 calls out to md5sum to generate an MD5 checksum for a file. This is a bit silly, as Python can do it natively, is probably a bit slower (since a new process is invoked), and may cause problems on platforms without md5sum (Windows, some embedded Linux systems).

Solution: replace it with a native Python solution.