Add links to the GitHub repository
Includes only index.js

Makes the npm package as small as possible by including only necessary files.
See http://www.rudeshko.com/web/2014/05/13/help-people-consume-your-npm-packages.html
Add repository URL

Reported by npm as a warning:
```
npm WARN package.json tilelive-http@0.2.3 No repository field.
```
Merge pull request #4 from julien-f/package.json

Package.json updates