Add mark as read module

Adds a "Mark as read" module which allows links to be toggled between read and unread.

Also added a reminder to `README.md` to increment the `settings_version` variable in `defaults.json` when adding a new module.

![image](https://f.cloud.github.com/assets/46863/933598/af64e5b8-0059-11e3-9503-e72485c061e7.png)

![image](https://f.cloud.github.com/assets/46863/934661/26e252aa-006f-11e3-8e21-d1d1ace55936.png)