Add an xpath evaluator script

[Adrian Wilkins]

Binary package hint: libxml-libxml-perl

libxml-xpath-perl supplies /usr/bin/xpath as a means to extract nodes using XPath
on the command line.

XML::XPath is very slow, so this script is too.

Patching this script to use XML::LibXML instead produces a > 10x speedup.

Very little effort is required as XML::LibXML tries hard to replicate the API of
XML::XPath

Test query run over an 8MB XML file :

/usr/bin/xpath : 30.8s real time
/usr/bin/xpath2 : 2.2s real time

Need I say more?

Enclosed my hasty port of the original script with a few comments.