Use models

I have tried to fix `git review list` and `git review show ID`.
While doing this, I wanted to avoid multiple HTTP requests for the same purpose while keeping a simple code. So I had the idea to do more in the model and remembering inside it what has already been requested.
I'm not sure that moving all HTTP requests to the model is a good idea but yet, I find it quite convincing.

However, it is a bit annoying to pass the host variable (`@github`) to every model. My current solution is with a `attr_accessor :host` which isn't pretty because I need to call `...host = @host` for each object creation that need to do HTTP requests.