generating Style names

I see you put a lot logic in the instance generator to automatically generate proper style names. ([Put width in style name instead of family name #35](https://github.com/googlei18n/glyphs2ufo/pull/35#issuecomment-210227192)). I don't think that is the right place to do this. The style names should be properly set in the .glyphs files already.

There are three ways to do that.
1. Do it manually in Glyphs.app. That might be difficult to get right as many people working on this files and not all of them might get it right.
2. Provide a script to run in Glyphs.app that sets the names correctly. That has the advantage that people testing from within Glyphs.app will get the same result as if generated from fontmaker.
3. Use a preprocessing script that adds the information to the instances but is independent of the actual interpolation code.

That has at least two advantages.
1. It simplifies the export script
2. The library would be more flexible and could be used to produce fonts with different requirements on style names.

So I think the export script should stick to whats in the .glyphs file and not trying to fix things.