Back-off on throughput throttling

Adds exponential back-off for requests that fail due to throughput limitations on your table. Dynalite doesn't pay any attention to throughput, so adds some very basic verification that throttling is happening to `/test/live.test.js`.

Also adds `ReturnConsumedCapacity` support to requests that are run through `dynamoRequest.js`. Refs #29 

@mick my last question before merge is: should throttling be default behavior, or should it only be implemented if specified in `opts`? See https://github.com/mapbox/dyno/blob/3d0620ef8c8127597486b7929694a4c3e8fa60ce/lib/dynamoRequest.js#L36-L38