Add examples (issue #5)

This version introduces multiple enhancements:

Xml usage documentation
fixed phpdoc in Xml library
added export as xml to base object doc

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

Added:      examples/xml.php
            -- Added --
                xml examples container

Added:      examples/xml/errors.php
            -- Added --
                error example

Added:      examples/xml/exist_file.php
            -- Added --
                load data from existing file or string

Added:      examples/xml/new_file.php
            -- Added --
                create new file using Xml library

Added:      examples/xml/source.xml
            -- Added --
                xml data example

Modified:   examples/base_object/export_data.php
            -- Added --
                xml data export example

Modified:   Data/Xml.php
            -- Updated --
                fixed phpdoc documentation