Added a new bower.json

I think it will be really interesting to have a bower package to implement this, and it's not really expensive.
The only thing missing after that is to register the package on bower `bower register d3-timeline git://github.com/jiahuang/d3-timeline.git` and create a 0.0.1 tag to make it easier to track `git tag -a v0.0.1 -m "release 0.0.1"`