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
Merge branch 'test-compatibility' of github.com:jonathantneal/picturefill into jonathantneal-test-compatibility
Pulling the `has` off this var, since we’re not strictly using it to check something.
Fixes tests in non-calc browsers: defaulting to 100vw (no calc() support, no `sizes` attr) now uses the offsetWidth of the document rather than the measurement el.
Probably this should use real JavaScript that actually works; I don’t know. I’m no expert.
Dist files.
Conflicts