2.0.0 release fixes (#815)

* Documentation / test cleanup
* Remove deprecated avoidMinZero option
* remove all references to dygraph-combined
* Drop for..of and polyfill to save space
* Check for strings in native format
* Generate minified CSS
* Get npm distribution working
* Guard process access for rollup
* Add ES6 example to home page
* Update tests to source CSS from dist
* update some references
* Drop bower, bump version
* index.es5.js
* bump version