Merge pull request #11 from SciRuby/master

Improved README. Added proposed copyright/license modifications. Added examples.
Define extra fields addded by Vega transforms.
Update README and add one more example.
Bug fix.
One more example.
More bug fixes.
One more example.

Not sure why it's not shown on bl.ocks.org. It works fine in Vega's
online editor.
Point examples to bl.ocks.org.