Fixed a bug occurring when no source media matched

Releasing v4.1.3 and fixing a bug that occurred then no source media
matched
Changed info about the no more optional `img` tag
More information about the `img` tag
Fixed a bug that occured on IE8. Now the src element for the img tag is not required anymore to the picturePolyfill work in IE8
Fixed an issue with _getImgTagsInPicture + tests

Tests are now all green in each browsers
Removed risky Array.prototype.indexOf from tests
Releasing 4.2.0
Merge branch 'master' into gh-pages