[hotfix] Add footer slide-in for anonymous users

# Purpose

This PR adds a "slide-in" style popup to the bottom of the OSF, when a non-logged-in user accesses the site.

Closes #1047

The verbiage on the popup has been approved by @saradbowman, as has the functionality.

# Changes

![slidein](https://cloud.githubusercontent.com/assets/131414/5344463/b711f5e0-7edd-11e4-9910-2f9d1794d7bd.gif)

HTML for the slidein was added to `base.mako`, but only if no user is logged in.

The panel slides in three seconds after the page loads. If the user dismisses the panel, a cookie is set to prevent it from re-occurring for 24 hours.

# Side Effects

The jquery.cookie plugin has bene added to `bower.json`; `bower install` must be rerun when this change is deployed.
+1 for `hidden-xs`