use window instead of this

to fix compatibility with browserify or webpack
add browser environment setting
add main field

to allow publishing to npm
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.
[+]: add a class, so that everyone knows where the helper-element comes from
Merge branch 'block-mixed-content' of github.com:zcorpan/picturefill into zcorpan-block-mixed-content
Closes #306 - Ensures entire <length> is chucked out the window upon encountering a percentage-based value, regardless of `calc()` usage, matching Canary impl.
make CI build faster
Merge pull request #300 from zcorpan/patch-2

Properly check for sizes support
Merge pull request #298 from zcorpan/patch-1

Update spec link and uppercase Contributing.md
make window resizing in IE8 work
Merge pull request #335 from thomastorfs/use-attachevent

make window resizing in IE8 work