Plugover refactor

Instead of running stand-alone, this application is now a set of Rack middleware that get injected to sgx-catapult. That means sgx-catapult is now a hard dependency (and is included as a submodule so we can easily manage the version of that dependency, etc). sgx-catapult's Gemfile is included in our own (though we can have extra stuff if we need it).

Basically, instead of running sgx-catapult at all, a user of this version of fwdnum runs only this instead, and points catapult callbacks for both messaging and voice to this one instance that will be augmented to handle both kinds of data coming in.

Additionally, everything has been refactored to not use blocking IO, since changes were large enough that I just went ahead and did that at the same time.

No behaviour should change at this point, but this will make future work (such as voicemail and ad-hoc commands to configure fwdcalls) much easier.