Add integration tests

Need something in place to enable refactoring.

Potential refactorings floating around in my brain:

* Toplevel object's responsibility is to just protect the lib from the big bad world (in this case, that's `Turingbot::Bot` protecting the rest of the lib from Rack). Everything else can exist independently of this, so probably pull process_message and channel logic and so forth out into another object that can be tested without sending a web request.
  - push channel checking down into `process_message`
  - extract `process_message` into an object, initialized with the name and channel (then can test/fix the regex -- e.g. if bot name was "Turing|Bot", and someone sent a message to "TuringAdmissions")
  - would probably rename Bot to App, to me it's just a way to wire the bot into Rack, but the bot itself should be independent of the fact that it's being served in a `Rack` app. I should be able to use it with full functionality from within another lib (ie I could bring it into apply.turing.io which could post messages to staff channel when someone applies), but right now, logic about when to respond is integrated with knowledge of rack
* `Turingbot::Command::URL` and `Path` should be injectible
* de-singleton `Turingbot::Command::Commands` (I should be able to run two different bots at once with different sets of commands). I don't really know what this looks like yet. Maybe fine to have a singleton CommandRegistry if running a command is decoupled from the global list. Or maybe the toplevel (our rack app) wires it all together. At the very least, I should be able to test its behavoiur without fucking up the object (https://twitter.com/josh_cheek/status/502665494596702208).
* I dislike the side effect of sending the message down in the command. General feeling is to stick that in its own object, and pull it as high as possible. I don't exactly know how high, maybe all the way up into Bot. Then I can then hit the bot and get back a list of messages it would send, and then choose to send them or do something else with them if I want (log them and then send them, send them and send myself an email of every message sent, compose bots out of other bots, etc)
This is wrong based on the fact that params_for checks for :from, not "user_name"