Async support

[pehrlich]

The controller call to impressionist relies on the request and other contextual data, with no apparent hook for asynchronous operation (e.g., through a worker).

Has anyone set this up before? How is synchronous performance?

It looks like making a custom version of this method in the controller called impressionist_async might make sense. Maybe there's a Rails4 api to make this super easy, but we're still on 3 so may not be as useful. https://github.com/charlotte-ruby/impressionist/blob/master/app/controllers/impressionist_controller.rb#L15

[sunny]

Duplicate of #140