topojson-merge

This isn’t ready to merge yet, but I’m opening this to track the progress.

It seems fairly functionality, but it doesn’t quite work:

![screen shot 2014-03-27 at 10 03 43 am](https://cloud.githubusercontent.com/assets/230541/2540334/f5405e2c-b5d1-11e3-8f7f-6842b3c93764.png)