add ufo2fdk.pens to 'packages' list.

previously, the pens module was being ignored when ufo2fdk was
installed via `setup.py install`. this causes it to be built and copied
along with the rest of the module.

this resulted in the following ImportError when calling `import ufo2fdk`

    ImportError: No module named pens.t2CharStringPen

![image](https://f.cloud.github.com/assets/216572/1302950/c73af9c4-3142-11e3-90c9-2efd535e5a6c.png)