Poster image to stretch both horizontally and vertically while staying centered and maintaining aspect ratio.

The existing poster implementation uses an <img> tag
stretched horizontally with capped max-height. This
works in some cases like pillar-boxing when the poster
is not as wide as the player. However it fails to
letter box when the player is taller than the poster.

This change uses a stretched div with background-image
and backround-size to create a vertically aligned
poster image which stretches in both directions to fit
while maintaining aspect ratio.

For browsers which do not support backgroundSize (IE8)
the original <img> based strategy is used as a fallback.