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.
fixed incorrect looping from length + 1 to length in parse descriptor, and removed unnecessary undefined check
Cache detection image
Merge pull request #346 from jonathantneal/cache-detection-image

Cache detection image
Safely get last character #296

Work around inability to lookup any character in a string by its index.
Merge pull request #347 from jonathantneal/safely-get-last-character

Safely get last character #296
Test compatibility

Prevents older browsers from failing when the single-argument
insertBefore method is used or when calculations can not be made on
elements outside the body
Repaint on src change #332

Resolves an issue with Safari browsers failing to display images with
the correct sizing
Code style fixes.
Merge branch 'jonathantneal-repaint-on-src-change'
Comment with issue related to `WebkitBackfaceVisibility` hack.
Merge branch 'test-compatibility' of github.com:jonathantneal/picturefill into jonathantneal-test-compatibility
Move picturefill length insert #340
Pulling the `has` off this var, since we’re not strictly using it to check something.