Implemented new transform for issue #238

- New HighlightAverages transform
- `ds.chart.util.get_palette()` will just return its argument if it’s
already a color array
- copy the `options` object in `chart.json()` so that the new
`item.clone()` returns a deep copy
Move palette generation to common utils
Merge pull request #240 from urbanairship/issue-238-highlight-averages-transform

Issue #238: highlight averages transform