Add EventDrop#created_at and drop (skip_)created_at from EventFormattingAgent.

As mentioned in [#410](https://github.com/cantino/huginn/pull/410#issuecomment-50548617), this PR adds EventDrop#created_at which is universally available in Liquid interpolation and obsoletes the skip_created_at option from EventFormattngAgent, leaving `"created_at" => "{{created_at}}"` in the default `instruction` setting to keep compatibility for users.
OK!  I put 'agent' here in an earlier PR for the same reason (compatibility), so you may probably want to remove that too.