Publish to npm

[ajhyndman]

This diff configures the package.json file for publication to npm.

• Adds the @dropbox-performance scope to the package name
• Adds a "module" entrypoint for ES2015 consumption
• Adds a "files" key to exclude all other files from the distributed artifacts
• Defines a "release" script using the library np (https://www.npmjs.com/package/np).

To release a new version:

$ npm login # authenticate with npm
$ npm run release -- [version]

To publish a test version (e.g. 0.0.2-2):

$ npm login # authenticate with npm
$ npm run release -- 0.0.2-2 --tag=test --any-branch