add new promise-based toImage and downloadImage to Plotly#446

Merged

etpinard merged 7 commits intoplotly:masterfrom

timelyportfolio:feature/snapshot-pull

May 16, 2016

Commits on May 11, 2016

Commits on May 12, 2016

painfully work through more IE bugs with snapshot
timelyportfolio
committed
reject with error in `svgtoimg` in IE if not `format:'svg'`
timelyportfolio
committed

Commits on May 14, 2016

fix promise check in toimage_test
timelyportfolio
committed

Commits on May 16, 2016