HSEARCH-726

Anyone interested at having a quick look at this? This is a very simple fix to allow faceting on collections (indexed embedded fields). 

I want to run some performance tests still and create some sort of baseline. This is the simplest solution for now, but we can do better even w/o using the native Lucene faceting via the separate category index. I  think that using dynamic faceting as described here - http://blog.mikemccandless.com/2013/05/dynamic-faceting-with-lucene.html - would be the next step. However, for that we still would need some more meta information from the user (via some annotation or another parameter on @Field). 

Thoughts?