Add `createSubFolders` option, default to `true`

I've got a PR (https://github.com/Stuk/jszip/pull/157) in for JSZip to add a `createSubFolders` option that will fix #29.  This pull request just sends that option (defaulting to true if not provided) along to JSZip in each call to `.file`.
Seeing as it's a new feature jszip should bump minor meaning 2.4.0.