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.
I was thinking about the iOS app @chriseidhof wants to write. If we do not share the secret the user would need to generate a secret in the app, copy it to the huginn instance and then authenticate via the app. 
If that does compromise the authentication security it might be a bad idea, but i thought that would just undermine the application 'authenticity'.