Follow Prototype: Users being followed and their activities

    Adds the ability to display members that the current user is following and their activites.
    Activity's icons are now displayed using css instead of an tag.

This pr was based on https://github.com/natgeo/modules-activitystream/pull/12/
Mati, we don't need the `getImage` helper, do we? Especially if we are going to use functions in the API mapper to manipulate tricky image URLs.