Ruleset updating mechanism to fetch, verify the authenticity of, and apply new ruleset databases

This pull request contains all of the commits I (Zack Mullaly) have made over the course of the Summer of 2014 to build a secure ruleset updating mechanism for my Google Summer of Code 2014 project.
There are still some details (such as a public key) that need to be filled in by whoever now is the primary maintainer of the project, but the feature should be functional once the TODOs are taken care of.
Why does this need to be a pref? You can directly get the addon version at runtime, ex: https://developer.mozilla.org/en-US/Add-ons/Code_snippets/Miscellaneous#Retrieving_the_version_of_an_extension_as_specified_in_the_extension%27s_install.rdf. Then there's no need to update this pref every time the extension is updated.