Update MutationObserver for JSPM example

[nashwaan]

Removed old reference of MutationObserver from previous doc (existed in JSPM example) and replaced it with the new shim:
Old doc instructed to install webcomponents/webcomponentsjs/MutationObserver.
But new doc uses mutationobserver-shim/MutationObserver. However, a trace of old shim existed in JSPM code.
Also, provided an example how to use the polyfill file in the webpack.config.js.

[AshleyGrant]

Looks good to me. @niieani or @jods4 would you mind reviewing the webpack config provided?

[jods4]

Yeah, sorry I wanted to tackle the polyfills subject for a while but I haven't gotten to do it.

Although it's intuitive, I don't feel good with that config because it's not the first thing that will actually run.
Aurelia Webpack plugin will add a few modules in front of that, notably aurelia-polyfills and aurelia-webpack-loader.

I'll try to do something about it soon-ish.

[EisenbergEffect]

@nashwaan There seems to be some contention over the Webpack configuration. I'm going to close this PR, but could you submit another one that just fixes the SystemJS usage and then we can work out the Webpack update separately? Thanks!

[jods4]

The just-released RC3 version of Webpack plugin has changed this unintuitive behavior.

The code in this PR is now actually working as expected, which was not the case in RC2.

[EisenbergEffect]

So, just to confirm. We should merge this now, correct?

[jods4]

As far as the webpack config goes, I have no objection anymore.