Implement Shoeboxed.authentication_url

Armed with the app name and callback url you can generate an authentication url:

```ruby
> Shoeboxed.authentication_url \
    :app_name => "Woot!",
    :callback_url => "http://foo.bar/shoeboxed/callback"
=> "https://api.shoeboxed.com/v1/ws/api.htm?SignIn=1&appname=Hoyt+Accounting&appurl=http%3A%2F%2Ffoo.bar%2Fshoeboxed%2Fcallback&appparams="
```