Separate API modules in jsforce

Browserify build a script which includes all modules related, and it tends to be very large footprint even if user doesn't use all of the feature.

It should be separated into several modules under `jsforce` namespace, and allow to load partially like following:

```html
<script src="/js/jsforce-core.min.js"></script>
<script src="/js/jsforce-streaming.min.js"></script>
<script src="/js/jsforce-metadata.min.js"></script>
```