BUGFIX: Keep ratio on resize

Make a function that keep the ratio between width/height when the element is
sized to fit the container.

Use native H5P 'resize()' to initiate the element resize, instead of custom
'setInterval()'.

Rename human-readable title of library to use the proper term 'embedder'.
Merge pull request #1 from Ellinokon/bugfix/keep-ratio

BUGFIX: Keep ratio on resize