Fix #12: automatically add binary executable "uva" when installing via n...

Hi lucastan,

Your uva-node tool is awesome. I found it on npmjs.org. However, nothing happened after install via npm install -g uva-node and it is because lost of `bin` option in package.json. Anyway, I fix it and add some explanation in README.md to show an even more easier way (for node.js users) to install the tool.

Please consider this pull request, and thanks again for your awesome tool :+1: 

-Milnex