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.
> Does everyone knowing the secret matter, if only an authorized app is redirected to with the verification token?

I think that you can run a man in the middle attack if you know the secret, but everything else should be oke (its been a long time since i read the OAuth specs so I am not sure about it).

> Does the requirement to enter the return URI in the doorkeeper application registration make the iOS app difficult? (Or is there a non-web-app flow?)

If we share the secret the user should just need the huginn url (the callback part could be added by the app)

> Perhaps users can just make an application, then click a link to self-grant a token, which they hand to the iOS app?

That would probably work, I know you can register custom URL schemes in iOS apps, we could use that either for the token itself or the application secret. 

I am wondering how we could deal with third party closed source applications that want to access a huginn instance, they would need to share their secret or have support for multiple, user defined, application secrets (don't know if that's possible).