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. 
I think this is on the hot path for request processing. Is that right? If so, looping through even a small list of domainExceptionSites is probably inefficient. I would add a big TODO to optimize this bit (after profiling).