Applies c14n to the xml input
    @param xml: str
    @return: str

def c14n(xml):
    '''
    Applies c14n to the xml input
    @param xml: str
    @return: str
    '''
    tree = etree.parse(StringIO(xml))
    output = StringIO()
    tree.write_c14n(output, exclusive=False, with_comments=True, compression=0)
    output.flush()
    c14nized = output.getvalue()
    output.close()
    return c14nized