Internal webpack build and fully-functional NPM binary

This release addresses several issues:

• #62 - internal webpack build has been added for the CLI. The browser-facing script is still using babel-node for transpilation but, since that code will go through webpack in the user's own build anyway, there's no need to bundle the scripts further.
• #61 - npm install tested and fully functional, including bundled huron binary.
• #59 - contribution guidelines have been added

Note: I accidentally published version 2.1.0 on the beta tag in NPM, so I wasn't able to publish over it in the latest tag. I've bumped the version straight to 2.1.1 to accommodate this (I'm still getting used to this!)