Since the select option is never passed through from the ActiveRecord layer, it's never used.  While it might be nice to support that, at present it's helpful to set select to 'all'.  This improves performance in many cases, avoiding another network hit to do 'get_item' for each result in a query.
Merge branch 'master' of https://github.com/lisad/Dynamoid
Merge branch 'master' into batch-get