Added ToFacets and ToFacetsLazily for LuceneQuery

To ease use of Facets within a LuceneQuery, I added the extensions of
ToFacets for convenience. Added additional tests around these new
extensions - all passing.

Was not sure how to get IAsyncDocumentQuery into  ( (
RavenQueryInspector<T> )queryable ) so did not implement ToFacetsAsync.
If you would like to comment and indicate the right path, I could add
that before you commit this pull request.