Add documentation (issue #6)

This version introduces multiple enhancements:

added Xml documentation
added documentation link into classes

Details of additions/deletions below:
--------------------------------------------------------------

Added:      doc/Xml.md
            -- Added --
                Xml calss documentation

Modified:   Data/Object.php
            -- Updated --
                documentation link

Modified:   Base/Register.php
            -- Updated --
                documentation link

Modified:   Base/BlueObject.php
            -- Updated --
                documentation link

Modified:   Data/Xml.php
            -- Updated --
                documentation link