Add NPM Module support (to use as Yarn package)

[arielj]

#18

This PR adds support to use this repo as an NPM/Yarn package so we can use it with Webpacker.

I implemented this so it uses the same source files as the Rails gem so any change in the CSS and JS applies in both cases. I didn't change the Rails gem related code this same repo can be used in both ways.

I had to create these similar-but-different js and scss files because the Rails gems dependencies and the node dependencies uses different names for the assets (bootstrap vs bootstrap-sass, material vs material-design-light) and different organization.

Changes:

• added index.js as the entry js point
• added index.scss as the entry scss point
• added package.json config file for npm with jquery, bootstrap and material-design-lite dependencies
• updated README with better instructions for both methods

I'm creating a PR in FastRuby.io too that applies this changes.
EDIT: https://github.com/ombulabs/fastruby.io/pull/231

[cleicar]

Looks good to me @arielj

[cleicar]

I tested locally and looks good! Thanks.