Observe mutations

Hey guys,

a few days back I twittered the following:

<blockquote class="twitter-tweet" lang="de"><p><a href="https://twitter.com/scottjehl">@scottjehl</a> Have you guys thought about adding Mutation Observers in order to react to script-induced changes to picturefill&#39;s sources?</p>&mdash; Christian Schaefer (@derSchepp) <a href="https://twitter.com/derSchepp/statuses/460874704602271744">28. April 2014</a></blockquote>
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<blockquote class="twitter-tweet" lang="de"><p><a href="https://twitter.com/scottjehl">@scottjehl</a> Maybe I can manage to get my act together and contribute with a pull request not too soon.</p>&mdash; Christian Schaefer (@derSchepp) <a href="https://twitter.com/derSchepp/statuses/460878413893742592">28. April 2014</a></blockquote>
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So here it is! `picturefill` supercharged with [Mutation Observers](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver). These things observe the DOM and detect any newly injected `picture` or `img` nodes and has the polyfill process them automatically. The same happens if someone changes the source attributes of existing images. Behavior-wise this brings `picturefill`a lot closer to native.

Older browsers that do not yet support Mutation Observers (see [support tables](http://caniuse.com/mutationobserver)) will use the older [Mutation Events](https://developer.mozilla.org/en-US/docs/Web/Guide/Events/Mutation_events). This includes IE9 and IE10. Mutation Events are only partially supported by Android 2.2 - 4.3 (only detects new nodes, no modified attributes). Feature tests [over here](https://github.com/mwbrooks/mutation-event-tests).

All other browsers, like <= IE8 fall back to the state we already have right now.

I really hope you like that feature addition!

Cheers

Christian