Feature: simplify PACKAGE_INFO implementation

I simplified the `PACKAGE_INFO` implementation by removing `lib/package_info` and using `require './package.json'` directly where needed. [Node.js implements this feature](https://github.com/joyent/node/issues/1357) since version 0.8, therefore I raised the node.js version requirement in `package.json` to '>= 0.8'. Really, groc's dependency on node ≥ 0.4.12 is astonishing old (:astonished:) and to my mind node < 0.8 is too insecure for production purposes.

Everything seems to work as expected, see https://gemnasium.com/sjorek/groc .
Are there any objections I'm not aware of ?

Hint: Upcoming features I'm going to offer as pull request(s) depend heavily on this technique.

Cheers & thanks,
Stephan