Update to use native 'picture' syntax; works with Picturefill 2.0

Fixes #38.

API change:
 - Remove 'picturefill' markup option; only 'picture' is valid for now
Merge pull request #41 from mattdsteele/picturefill-2

Update to native <picture> syntax (Picturefill 2.0)
Reference new syntax and Picturefill 2.0 usage
Merge pull request #42 from mattdsteele/picturefill-2

Reference new syntax and Picturefill 2.0 usage
Apply HTML attributes directly to image.

These were lost in migration to PictureFill 2.0
Merge pull request #47 from loksland/patch-1

Apply HTML attributes directly to image.