Add a license field to package.json.
Add a license field to package.json.

Currently npm install warns with the following message:

npm WARN package.json plottable.js@1.4.0 No license field.

Plottable is MIT licensed, according to the LICENSE file, and I don't
like this warning message.

palantir/plottable,iobeam/plottable,danmane/plottable,RobertoMalatesta/plottable,softwords/plottable,NextTuesday/plottable,softwords/plottable,iobeam/plottable,palantir/plottable,danmane/plottable,alyssaq/plottable,alyssaq/plottable,jacqt/plottable,NextTuesday/plottable,RobertoMalatesta/plottable,RobertoMalatesta/plottable,softwords/plottable,palantir/plottable,palantir/plottable,gdseller/plottable,gdseller/plottable,jacqt/plottable,iobeam/plottable,onaio/plottable,danmane/plottable,gdseller/plottable,jacqt/plottable,alyssaq/plottable,onaio/plottable,onaio/plottable,NextTuesday/plottable