Implemented binding quote_attrib() and quote_xml() methods as they are in python-stix. Added unit tests
Updated all str() and unicode() calls on self.valueOf_ to use quote_xml() instead.
Removed utils.escape() and utils.unescape() methods.
* Changed normalize_to_xml() and denormalize_from_xml() methods
  to not call escape() and unescape() methods as the lxml library
  and bindings take care of unescaping and escaping.
Merge pull request #223 from CybOXProject/unicode

Unicode