Add a Promise polyfill to the demo so that it works in IE11
Add a Promise polyfill to the demo so that it works in IE11

Connects to #751

sourcelair/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,akalipetis/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js