Added a xml script to merge two xml constructively.

This is a script written in php which will allow one to constructively merge
two XML files i.e. the fields common in both the XML files will be updated
with the values from the old XML file into new XML file. But the fields which
are not available in old XML file but are present in new XML file will remain
intact thus merging both the old and new XML files constructively.

Many a times while working with XML files we come across a need for a script
which will merge two XML files constructively and so I thought of creating
such a script in PHP which will enable merging of two XML files into one XML file.

Before executing, the New XML, Old XML & Schema XSD file variables needs to be
updated in the script file as per your need. Currently, the script doesn't
support passing of XML file names as command line arguments.

The script can be executed by command "php xml-merge.php"

Signed-off-by: Samarth Parikh <samarthp@ymail.com>