Universal Module Definition - commonjs - amd - browser

Hi Lea, this pull request is about nodejs usage. 

*Changes:*

- umd for prism-core
- commented placeholder for plugins, so that download.js will insert selected scripts inside umd factory
- package.json - login at npm and call ```$ npm publish``` (eventually make some changes in it)
- minified sources with uglifyjs

Hope it will be useful and  of course, all questions will be answered eagerly. Cheers.