type inconsistency

https://github.com/level12/webgrid/blob/master/src/webgrid/types.py#L136

Types for grid spec indicate list of dicts for columns and column types. The [JSON renderer](https://github.com/level12/webgrid/blob/master/src/webgrid/renderers.py#L194), though, produces a dict for each, which I believe is the correct output.