Adding queries on index and fixing limit bug.

I've just started using this branch, as I'm migrating part of my backend to dynamodb. I appreciate you sharing this!

Two fixes in this pull request. One was the Limit query, which was confusingly documented in AWS, but I had to change the type for it to work (as your comment suggested).

I also added two QueryOnIndex methods, as these are necessary to make queries use a Local Secondary Index.

Hope this is helpful.

p.s. the whitespace change was just a result of gofmt