Added an encoding parameter to to_xml() and from_xml() methods. The to_xml() method can return a unicode string or any encoded string.
Added unittest for utf-16 roundtrip api-xml-api
save_encoding() contextmanager sets the cybox.bindings.ExternalEncoding
Changed default encoding arg on input/parse methods to be None
Modified to_xml() docstring.
Modified tox.ini and travsi.yml to point to unicode branch of python-cybox
Set default encoding to None in parse_xml_to_obj()
Merged origin/master. Still pointing to unicode branch of python-cybox
Attempt to address the travis build error by adding python-cybox/unicode branch dep
Use master branch of python-cybox now that the unicode branch has been merged in.

[ci skip]
Merge pull request #235 from STIXProject/unicode

Unicode: Round Two