Declare jszip as a module, in addition to the JSZip global

I had a require.js (`--module amd`) powered project, and was having trouble getting require.js to play nicely with JSZip as required by a TypeScript file. When JSZip sees that AMD's define global is present, it doesn't export as the global `JSZip`, but the `jszip.d.ts` previously only supported global-style consumption. After some head-banging fun with TypeScript's undocumented `/// <amd-dependency path="lib/jszip" />` feature, and tricksy `jszip.d.ts` shims, I realized the reason why TypeScript didn't want to compile my `import-require` (with a one-two punch of TS2306 + TS2307) was because there was no `declare module ...` bit in jszip.d.ts.

I feel that not declaring the global would be the more proper solution, but to retain the widest compatibility possible, I only added the `declare module "jszip {...}` bit.

**References below:**

How jszip autodetects the AMD/commonjs/`<script>` handler:

    function(e) {
      if ("object" == typeof exports && "undefined" != typeof module)
        module.exports = e();
      else if ("function" == typeof define && define.amd)
        define([], e);
      else {
        var f;
        "undefined" != typeof window ? f = window : "undefined" != typeof global ? f = global : "undefined" != typeof self && (f = self), f.JSZip = e()
      }
    }(...)

[AMD style](http://stuk.github.io/jszip/documentation/examples.html#toc_1)

[CommonJS style](http://stuk.github.io/jszip/documentation/examples.html#toc_2)