Dataset metadata

Adds a metadata record to Dynamo for each dataset. Tracks:
- count: number of features
- size: data "size", defined as sum of `JSON.stringify(feature).length` for each feature
- extent: west, south, east and north bounds. Does not contract when features are removed.

Adds one new function to cardboard's API: `cardboard.getDatasetInfo(dataset, callback)`

~~Blocked by https://github.com/mapbox/dyno/pull/14~~, and should probably land ~~after [cardboard.update is merged](https://github.com/mapbox/cardboard/pull/45)~~.

cc @mick, @willwhite