Migrate from Browserify to Rollup

[mikeybanez]

We started out with Browserify to be able to use require() in the browser, but now, better library bundlers like Rollup are available. In order to maintain good compatibility with newer app architectures that use JS modules, we will want to migrate to Rollup.

Please note the following use cases we still want to support:

• We need a standalone build that can be added through <script src> for simple plugins. In this case, xjs can simply be injected a global variable (instead of working through require('xjs'))
• we need a umd build for npm, that can be imported through import xjs from 'xjs-framework'

For simplicity, we should not yet support apps that build directly from our source through TypeScript preprocessors.

[mikeybanez]

I created a 3.0.0-alpha milestone for this; we can test out our npm package with -alpha- version numbers. The main breaking change here is probably for the standalone build. We should ready up some documentation changes here.