Attach facets to search result objects

This is for issue #179.

If this looks legit I can add tests and docs.

Testing locally this allows me to do something like this:
```
res = S().query(...).facet(...).highlight(...).values_dict(...).execute()
# Get the raw results including highlights.
res.results
# Get the facets w/o making another HTTP request.
res.facets
```