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.
This looks like the manifestation of the last problem I was talking about a while ago. You can't write to files in the extension's base path, which is something I was told in the extdev IRC channel and verified and reported in my emails to the mailing list. The solution was to use this constant, OS.Constants.Path.profileDir, which should always point to a valid directory within which  we can reliably and appropriately store the downloaded ruleset database files.