convert global.pp class to a defined type

This fixes the phantomjs::global resource issue that people are running into (https://github.com/boxen/puppet-phantomjs/issues/8) by converting the resource to a defined type in line with how the phantomjs::version resource is structured.

It also sets the $version parameter to the $title so that the usage example given - phantomjs::global { '1.9.0': } - succeeds. 

The spec passes with a little modification.