Adds ability to sort backwards when using DynamoDBContext.

[duckmaestro]

Useful because otherwise you must form the query by hand, but to form
the query by hand you must (often in practice) know all of the column names you wish
to query, but in the current SDK you cannot (immediately) leverage all
the attribute tagging in your model classes to easily have a List
of columns to retrieve.

[PavelSafronov]

Thanks for this fix! I will review this change and will update the pull request when/if we merge this.

[PavelSafronov]

We will be adding this functionality (backward query) to the SDK and it should go out with our next release.
Our change will be slightly different from the suggested fix, though, to more closely match other parts of the API.

[duckmaestro]

Great, thank you.