Adds doorkeeper to allow application to authenticate against huginn

This should help with #626, it also has the benefit that users do not need to share their username and password with any third party application that needs to access the huginn instance.

Since we probably can not keep the application `secret` secret I think we should automatically add every known application that is able to use huginn via a migration.

At the moment you need to access `<your huginn instance>/oauth/applications` and add the application manually.
You are right about the performance issues when using bcrypt one every request, I did not think about that. When we issue expiring token, would you also provide a way to renew them via the API or just use them for services you just want to use for a limited amount of time?