xvfb-run and Test::NeedsDisplay

As suggested here:

http://blogs.perl.org/users/minty/2013/03/gtk3webkit-on-ubuntu.html#comments

I've patched the Makefile.PL and t/webkit.t to use Test::NeedsDisplay

For me, it both makes things pass without needing explicit use of xvfb-run, and also emits fewer warnings.

I've updated the POD with an Installation section as suggested and listed the debian packages I needed to install on Ubuntu 12.04 to make things work.

I confess I'm not sure how one goes about detecting if the GIR file is installed :/