Change the entrypoint for the Horizon client in package.json

[mglukhovsky]

The entrypoint for the Horizon client is currently lib/index.js, which is the unprocessed Webpack configuration, whereas it should probably be dist/horizon.js.

We should also figure out why lib/index.js contains unprocessed directives.

[deontologician]

After sorting out the webpack stuff, it turns out lib/index.js is the entrypoint we want. The major issue was that we had a fetch directive in a require statement which is bad.