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.
If dbHash is empty, it won't equal `hash`, and the callback will return after any error reporting.
Also, if you meant "if `arrayBuffer` is empty", then it will return right away since the test on line 186 will only pass if arrayBuffer contains data, and all of the code to handle that data is run provided that test passes.