2.2.0 Stable.
Updated contributing workflow/notes, for the team.
Update CONTRIBUTING.md
Updating package.json so NPM is happy.
Regen dist
allow getting `srcset` in Safari using the data-pfsrcset attribute.
Added note on validating markup with shortest possible `src` data-URI.
Versioning details in README.
Merge pull request #381 from aFarkas/issue-338

allow getting `srcset` in Safari using the data-pfsrcset attribute.
Fixes the `console.warn` error in IE8.
Merge remote-tracking branch 'upstream/master' into alternative-images

Conflicts:
	dist/picturefill.js
	dist/picturefill.min.js