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'd suggest the initial version could just allow you to make any number of API tokens, each with a sufficiently-secure random word name, and an optional expiration date (`nil` means never).  You can give these to apps you trust, and time limit them if you'd like.  You can also revoke them at any time.  These are basically oAuth access tokens, except the recipient doesn't need to register an app since we want to support iOS apps, etc.