Update build instructions with grunt and uglifyjs details
Allow multiple renders to occur simultaneously
Increase mocha timeout for multiple-render tests
Temporarily add manual update to es6-promise polyfill

This replaces an old and outdated Promise polyfill that was included
with previous versions of html2canvas, and which consistently overwrote
native browser implementations when it shouldn't have. It is essentially
a (slightly modified) paste of the most current minified version of
es6-promise, found here:
https://es6-promises.s3.amazonaws.com/es6-promise-2.0.1.min.js

The plan is to eventually include this polyfill as a submodule
dependency, but the build process for es6-promise is a bit
cumbersome/opaque/painful at the moment, so this manual update was an
easier interim solution.

Please note that I had to manually add the actual call to
`ES6Promise.polyfill()` to load the polyfill--this will become
unnecessarry in future versions of es6-promise. Note also that I had to
manually pass `window` to the polyfill's IIFE, since `this` is an empty
object in the current scope.

Ref niklasvh/html2canvas#494.