Allow boost fields to be indexed and stored

Currently there is no way to change the default behavior of boost fields being not indexed and not stored. If a boosted field can be also indexed, then it can be sorted/faceted on if there is a use case that requires it. This change will make boost fields behave like the timestamp field, which can be indexed and stored.

I know that Shay is not a fan of document-level boosts and in fact, Lucene 4 has gotten rid of them. But the boost field feature is there, so it would be great to use it to its full potential.