Feature/issue 165

Adds functionality that allows the user to whitelist certain potential trackers which need to be whitelisted to work. E.G. Youtube comments. This feature is already present in the chrome version but this would add it to the firefox version. CC @jsha for review. 
This doDependingOnIsPrivate function is rather odd in both name and function. Why does it take a callback? Is it expected to be async in future? If not, a more natural approach might be a function like getStorageForContext that returns a storage object.

(Not a change for this review, just a drive-by comment)