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

Merged

etpinard merged 7 commits intoplotly:masterfrom timelyportfolio:feature/snapshot-pull

May 16, 2016

+569-91

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