xml data handling (issue #2)

This version introduces multiple enhancements:

xml data object (extending DOMDocument)
xml data handling by Object

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

Added:      Data/Xml.php
            -- Added --
                hml data handle

Modified:   examples/base_object/set_data.php
            -- Added --
                examples with xml

Modified:   Base/BlueObject.php
            -- Added --
                usage of ClassKernel\Data\Xml, DOMException, DOMElement
                $_integerKeyPrefix
                _appendXml
                _xmlToArray
                _stringToIntegerKey
                _convertArrayDataToXml
                _appendDataToNode
                _integerToStringKey
            -- Updated --
                __construct handling of simple_xml
                toXml added logic to create xml data form oject
                _appendXml renamed to _appendSimpleXml
                _arrayToXml added logic to convert data to xml