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.
True, but if we share the secret, anyone will have full access to your huginn instance, right?  I think we have to have the user make a new Huginn Application and enter the public and private keys into the iOS app?