`img` inside `picture` now mandatory

The img picture inside the picture tag is now mandatory. The script was
adapted to this assumption, and some functions were removed.
Just some indentation :)
Managing both `src` and `srcset` in the `img` tag

picturePolyfill now manages the case in which the `img` tag contains
both src and srcset, and it extracts both `srcset` and `src` from the
`source` tags, applying it to the internal `img` tag
Added tests, fixed some bugs