Added attribute removal in SimpleXMLElement
Added canonicalization support to SimpleXMLElement


Uses c14n.py pure-python implementation (from SZI/W3C wstools et al) as a workaround if lxml (libxml2) C library is not available, or fails (for example, due deprecated features)
Use internal c14n if lxml not available/disabled in xmlsec
Add XML canonicalization pure-python implementation from wstools