add JSZipUtils.getBinaryContent

This pull request reuses a lot from JSZip : the grunt build, the license, the code.
As said in stuk/jszip#61, `getBinaryContent` comes from the file `browser-test-utils.js` of JSZip's unit tests. The documentation uses jekyll to generate the html pages from markdown files (in the documentation folder), a preview can be seen [here](http://dduponchel.github.io/jszip-utils/). I'm not really good for user interfaces so the documentation have a lot of room for improvement !