source src attribute warning
adding a check for console undefined
Fix demo markup.
Closes #215 - Find all `img`, look for `picture` parent, and parse `source` elements *until* reaching the `img` that triggered everything. This matches the invalid case outlined in the note on http://picture.responsiveimages.org/#update-source-set .
Properly polyfills the extended `srcset`, assuming the default `sizes` value of `100vw`.
Merge branch 'sourcesrc-notification' of github.com:jcutrell/picturefill into jcutrell-sourcesrc-notification

Conflicts:
	tests/index.html
Editing warning message.
Merge branch 'jcutrell-sourcesrc-notification'
Version bump.
Callin’ it: 2.1 stable.
One of these times, I’ll manange to remove all references to the prior version in time for a release.
Insignificant comment and code style cleanup
updated `Picturefill JavaScript API` in index.html
residual version change
Skip non-`img` nodes when using `options.elements`

Prevent processing when `options.elements` contains non-`img` nodes.
Update spec link and uppercase Contributing.md
Properly check for sizes support

`w.HTMLImageElement.sizes` is bogus -- there's no sizes property on the interface object. Per spec, it should be on the prototype, but that's not widely implemented yet (in some browsers it would go on the instance instead).
Block mixed content. Fixes #299
Avoid creating two img elements
Ref. #264 - Invalid example markup.