Remove zipper, async, temp deps, replace with node-zip.

I have some plans to update and incorporate a few new features into this project (column widths, data types outside of string/number, more flexible API) and I wanted to start with the quick hits first.

I will be changing some of the API as I go so I wanted to contribute this back before I start breaking existing programs built on node-xls-writer.

This commit removes the temp, async, and zipper dependencies (zipper is a native dep, which can sometimes introduce problems on some platforms). Instead we use the JS-only `node-zip`, a port of [JSZip](http://stuk.github.io/jszip/). The bonus here is that we can build the zip in memory and avoid all the filesystem accesses, speeding up the file generation and eliminating all the fs writes that necessitated the `async` dependency.

This is likely the last PR I'll contribute before I start changing the API - if you are interested in receiving those PRs as well please let me know and I'll send them upstream.