feature: round-corners for bars in barchart
Merge branch 'feature/rounded_corners' of https://github.com/sudodoki/morris.js into sudodoki-feature/rounded_corners
Tidy up.

- The options namespace should stay flat - no nested config values.
- Used single array literal instead of chained Array.concats.
- Added default values to config defaults, instead of using default
  method arguments.
- Tweaked spec.