Single-file 'System' for browsers via npm, bin/BrowserSystem.js

[johnjbarton johnjbarton]

I changed the way traceur works in the browser.  I believe that the bootstrap.js solution still works, 

but for convenience of experimenting users, now bin/BrowserSystem.js includes compiler, polyfills, 

System, and WebPageTranscoder.

In addition, we  traceurOptions attribute from <script type=module traceurOptions="--asyncFunctions">.

The wiki page is updated for modules:

https://github.com/google/traceur-compiler/wiki/LanguageFeatures#modules

jjb