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 believe that a user authorizes an app and is redirected back to a return URI (which must be known) with a verification token, and the receiving application sends its secret, along with a verification token, to the huginn oauth endpoint and receives a long-lived access token.

So, two questions:

* Does everyone knowing the secret matter, if only an authorized app is redirected to with the verification token?
* 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?)

Perhaps users can just make an application, then click a link to self-grant a token, which they hand to the iOS app?