Versioning

Closes #34

This is highly inspired from the way API versioning works on the Heroku Platform API to get it on pliny.
In `router.rb`, we now use `Pliny::Router` insead of `Sinatra::Router`.

This allows us to do things like this:

```
use Pliny::Router do
  version '1' do
    mount Endpoints::V1::MyApiEndpoint
  end
end
```