Abort installation if PhantomJS is installed

Hi,
PhantomJS is available on FreeBSD through [ports](http://www.freshports.org/lang/phantomjs).

However, installing PhantomJS via npm is currently impossible because OS is not in the list of supported ones (and that's reasonable as there is no downloadable archive for it on http://code.google.com/p/phantomjs/downloads/list).

In order to allow FreeBSD users install packages (like grunt-lib-phantomjs) that depend on this package, I suggest aborting the installation with NO_ERROR code if PhantomJS is already installed in the system.