An implementation of rolling table of contents

This pull implements a rolling table of contents, in which the three most recent articles are featured prominently, and all other articles are divided into their sections and displayed there.

This code was originally written in OJS 2.2.3, and has not been heavily tested in OJS 2.3.4, though this work is currently ongoing.

For a live example, see http://openmedicine.ca

tarek : )