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.
Sorry, was at RubyConf this week and am just catching up.

> 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).

It seems like maybe oauth2 isn't as good as I was thinking for Huginn, because what we mostly need is just token-based authentication.  Maybe all we really need is a page where you can make any number of API tokens, each with a memorable [random multi-word secret](http://preshing.com/20110811/xkcd-password-generator/), optional expiration date, and name.  Then you can hand one of these to any app along with your Huginn URL?