Improved auto-fetch of PhantomJS if absent

Now using Arquillian Phantom Driver that fetches PhantomJS from a maven
repo, instead of requiring a direct download. This should help people
behind proxies, etc.

This new driver also allows for local PhantomJS installations to
override the fetched one, so people who have PhantomJS on their path,
for example, will see that one used instead of a downloaded one.

The test code itself requires a small change to include the
ResolvingPhantomJSDriverService, responsible for firguring out whether
to fetch a PhantomJS instance, or use an existing one.
Merge pull request #57 from andyclark/feature/alternative-phantom-js-fetcher

Improved auto-fetch of PhantomJS if absent