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 think the user should just give a name / nickname for a new token (e.g., "Huginn iOS app key") and it should automatically generate a memorable but secure API key.  Something like GUID -> sufficiently-secure string of memorable words, like the XKCD generator does.