Added component.json to support resolving dependencies via bower

I needed to use topojson in a browser context - in a d3 map. My project was using bower and I thought it would be nice to make it work out of the box with it, instead of having to specify the full path to the JS in git manually.

If you decide to merge this pull request, please consider registering topojson with bower so it can be searchable. I saw you already registered d3, so I won't go into more details.

Thanks for these great tools!