17397: Timeline Feature [Preview]

Sharing a preview of the timeline feature.  The implementation in this pull request is designed to minimize the changes to the codebase.  This checkin is to get initial feedback and not to merge.  There is follow up work like:

- Move strings to strings_english.txt.
- Move styles to default.css.
- Move apis to core/timeline_api.php

The feature is completely stateless - i.e. no db schema changes or storage in the db.

Looking forward for feedback.

![timeline2](https://cloud.githubusercontent.com/assets/6446/3143453/38e7cde2-e9fb-11e3-8fd8-89a9e466fdb5.png)