Support solr field collapsing/result grouping


![screen shot 2013-09-11 at 10 40 10](https://f.cloud.github.com/assets/111218/1125013/3ae8270c-1b09-11e3-8b56-63c7c5eb3355.png)

As it stands, we're forcing the implementor to configure solr appropriately, and add a hint to Blacklight about how to handle the groups.

Here are some solr request parameters to enable grouping on our demo index:

```ruby
      :group => true,
      :'group.field' => 'pub_date_sort',
      :'group.ngroups' => true,
      :'group.limit' => 3
```

And the Blacklight configuration hint to make Blacklight use it (or, if there's only one group.field defined, we'll do the right thing and just use it):

```ruby
    config.index.group = "pub_date_sort"
```