Add support for custom CDN URLs with phantomjs

This fixes #41, and provides a way of supplying custom CDN patterns
on the comamnd line with a new "--cdns" option. Implements a
bare minimum "preferences" implementation to avoid changes anywhere
but in the PhantomJS specific yslow.
Fix some small jslint issues
Merge pull request #79 from marcins/feature-phantomjs-cdn

Add support for custom CDN URLs with PhantomJS