HTMLImageElement is the constructor not the prototype nor the element (should also fix: https://github.com/scottjehl/picturefill/issues/326)
fix image error in iframe in oldIE
make sure picturefill is evaluated 40ms after "last" resize event instead of throttling it to 60ms (better performance for slow resizes)
do not recreate small short living resize function with every resize, cache it instead
don't use the string true in boolean content attributes, people might be confused
alter some statements for performance (+ add console.warn check)
make sure that type support isn't running multiple times
fixes #303 and #304
add widthCache + cleanup div
re-factor image support detection + adjusted tests