Content server selection, user-defined whitelists, remove deprecated functionality, switched to mutation observers

This pull request brings the Firefox extension to the same level of functionality as the Chrome extension, including:

* Added user selection of content server. Previously the options pane only allowed the user to select from the list of official servers.
* Added user-defined whitelists. This is a long-needed feature that allows the user to define which domains they trust to not track their requests.
* [Removed Deprecated passive messaging system](http://www.privly.org/content/chopping-block)
* Mutation Observer:  
Here I updated the privly.js module to use Mutation Observers instead of mutation events. This has significant [performance improvements](https://hacks.mozilla.org/2012/05/dom-mutationobserver-reacting-to-dom-changes-without-killing-browser-performance/). The only bad part of this pull request is that Mutation observers are [relatively new](https://hacks.mozilla.org/2012/05/dom-mutationobserver-reacting-to-dom-changes-without-killing-browser-performance/) so we will be dropping support for some older browsers.