update main to be the correct file
update main to be the correct file

Also, set specific directories (via `files`) for installation into `node_modules`. I assume `visualizations` is required, but not sure. See [here](https://docs.npmjs.com/files/package.json#files) for more explanation.

As a nicety, I added the `build` script that will run the default gulp task if you execute `$ npm run build` on the command line.
521dimensions/amplitudejs