add configurable search result  columns

@maddalab @Primer42 @DanSimon This allows search result fields to be configurable. You specify a list of meta attributes (i.e. pool or secondary_role for example) and reserved attributes (created, tag, updated, etc.) and columns will magically appear in your search results.

```features.searchResultColumns = [ TAG, HOSTNAME, PRIMARY_ROLE, POOL, primary_role, secondary_role, nodeclass, RACK_POSITION, STATUS, CREATED, UPDATED ]``` will change the headers displayed to look like this:
![screen shot 2014-09-24 at 4 14 14 pm](https://cloud.githubusercontent.com/assets/130194/4395046/6095abbc-4427-11e4-86cb-0abbb3760b10.png)


Caveat: after discussing with @DanSimon, it is not possible to sort on arbitrary meta attributes, because by default we create their SolrKey as multivalued and not sortable in Solr (multivalue and sortable are mutually exclusive). If you click the header for a attribute you created thru the API without sortable set, you will see ```Error executing search: Cannot sort on SECONDARY_ROLE```. (Related: https://github.com/tumblr/collins/blob/master/app/collins/solr/SolrKeyResolver.scala#L114 and https://github.com/tumblr/collins/blob/master/app/models/AssetMeta.scala#L40)