Change home page Featured sets design.

Closes #505 

![home-page-features](https://cloud.githubusercontent.com/assets/96776/3622062/1f39a484-0e26-11e4-9f85-badde1f11f6d.png)

![home-pge-responsive](https://cloud.githubusercontent.com/assets/96776/3622064/224d9dd8-0e26-11e4-9d2d-bfab8abd4c5c.gif)

To explain why:  these images are decorative and don't carry additional information, and they aren't clickable, so the screen reader doesn't need to know about them. Blank alt makes them more or less invisible. However, they are images that may have copyright implications, and the library website convention is to add the caption/title and credit to the title tag. 


On Jul 17, 2014, at 9:50 PM, Jessie Keck <notifications@github.com> wrote:

> In app/views/catalog/_home_page.html.erb:
> 
> > @@ -1,6 +1,39 @@
> >  <% @page_title= t 'blacklight.home.title' %>
> >  
> >  <h1 class="sr-only">SearchWorks</h1>
> > +
> > +<div class="row features">
> > +  <div class="col-xs-4 col-md-3">
> > +    <div class="thumbnail">
> > +      <%= image_tag('digital-collections.jpg', alt: "", title: '"Cesar Chavez, Friday night meeting at Filipino Hall, Delano CA organizing briefing, 1970." Fitch, Bob (Photographer)') %>
> @jchristo4 the explicit blank alt and title attributes are from @jvine's spec in #505 (comment)
> 
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