Support for offset + limit pagination

[nicbou]

A common pagination style for REST APIs is offset + limit paginations. Instead of doing "page 3 at 5 items per page", you say "5 items after item #15". It's a slightly more flexible approach.

Looking at the code in pagination.py, I see that in the end, we turn the page number into an offset anyway, so wouldn't it be a better idea to allow an offset to be specified directly?