Add useGlobalPhantom task option for environments that can't use phantomjs binaries

The phantomjs package used to install phantom locally only installs a binary for the host system. On systems that can't support phantom binaries ([CentOS <5.8](http://phantomjs.org/download.html#linux) -- unfortunately out of my control), any task that uses `grunt-lib-phantomjs` fails. I've opened an [issue](https://github.com/Obvious/phantomjs/issues/29) with @Obvious to see if there's any hope of having phantom built from source in those cases, but adding a `useGlobalPhantom` task option to `grunt-lib-phantomjs` seems like the easiest path for now.

This commit adds `useGlobalPhantom` as an option available to tasks, which, when set to true, will use the `phantomjs` already in the system path.