Added Extensions ToFacets and ToFacetsLazily to LuceneQuery - See note below about ToFacetsAsync

To ease use of Facets within a LuceneQuery, I added the extensions of
ToFacets and ToFacetsLazily for convenience. Added additional tests around these new
extensions - all passing.

Was not sure how to cast IAsyncDocumentQuery as ( (
RavenQueryInspector<T> )queryable ) so I did not implement ToFacetsAsync.
If you would like to comment and indicate the right way to get IAsyncDocumentQuery into RevenQueryInspector<T> or some alternative, I could add
that before you commit this pull request.