Created xshelper to simplify common perlxs tasks

Instead of having to rewrite the full task of retriving Perl values
from hashes etc. Have encapsulated this into a separate header that
can be included into the main XS source.

Updated makefile to include newly created sources.
Added method to select udev subsystems

Started moving towards allowing device list retrival settings to be
included as a Perl hash. Now subsystems can be provided to search
for devices under that subsystem.
Adding support for udev system attributes

A hash of attributes and their values can now be provided when
searching for devices to further limit retrieved results.

Updated test.pl to check basic use of attribute functionality.