Adds property to XML_ToolsMixin that returns XML Header

* cafe/engine/models/base.py -- Added constants _XML_VERSION & _ENCODING with defaults of '1.0' & 'UTF-8' respectively
* cafe/engine/models/base.py -- Added property method xml_header which returns a formatted string to match the xml declaration header based on _XML_VERSION & _ENCODING

Change-Id: Id1a65c8d9ab8533c9205d3ba2d2c559f189d1891