Add support for new API: /signatures/:eventId?count=%d&offset=%d

Issues copied from https://github.com/rogerz/signature-api-legacy-for-angular/pull/1

> @rogerz

> Hi Roger, I didn't bring this up before, but we have a new API that we are using as well, so I'd like to add this API. I am not sure if you want to merge it to master or to put it in a different branch. There is a chance we will use both, but most likely it will be this new API.

> The new API is as follows: `/signatures/:eventId/?count=%d&offset=%d`. 

> Offset replaces `last_id` and `count` is the number to return at a time. If my changes are incorrect, please point that out to me. AFAIK, there is no need for the `directory` parameter.