No explicit dependence on libxmp anymore.  Back to ElementTree.
Merge branch 'issue104' of https://github.com/quintusdias/glymur into issue104

Conflicts:
	glymur/_uuid_io/Exif.py
	glymur/_uuid_io/__init__.py
	glymur/jp2box.py
	glymur/test/test_jp2box_uuid.py
	glymur/test/test_jp2k.py
Fixing tests broken by bad merge.
Removed restriction on only writing XMP UUIDs.  #104