Added bower support, ergo compiled JavaScript files, CONTRIBUTING doc.

Because bower requires that JavaScript codes be compiled to a single script, rather than built with a Makefile, I have included the compiled forms of d3-geo-projection. This appears to be the way that d3 itself is managed on GitHub and its own bower configuration.

This also required that I change the .gitignore to not ignore those compiled files anymore. In order to note this more formally, I have added a CONTRIBUTING file that explains that one should not edit these files directly.

This closes #7 pending a `bower register` by the maintainer (@mbostock) after merging.