md5sums a file, returning the hex digest

            Parameters:
                - f     filename string

def md5sum(self, f):
        ''' md5sums a file, returning the hex digest

            Parameters:
                - f     filename string
        '''
        m = hashlib.md5()
        fh = open(f, 'r')
        while 1:
            chunk = fh.read(BUF_SIZE)
            if not chunk: break
            m.update(chunk)
        fh.close()
        return m.hexdigest()