Limiting number of simultaneous PhantomJS instances

Thanks for the task: basically works great. I will likely use it as a basis for some other stuff I am working on as well, and may backport things if they seem like they might be useful.

This initial commit makes the command async by default, which seems like a good option rather than discovering a limitation by accident. Hopefully this cleans up #10.

By switching to `grunt.util.spawn`, this can now be used inline with, for example, [grunt-image-resize](/excellenteasy/grunt-image-resize), which can sort of take care of #4.