Prevent image distortions in IE. Fixes #62
Prevent touch emulation on video elements.

Calling preventDefault on the mousedown event prevents interacting with
the controls of HTML5 video elements in IE.
Added clickToPlay options for the YouTube and Vimeo plugins.

This commit solves two problems:
* YouTube/Vimeo only count views that have been started via the native
play buttons.
* Users who disable Flash by default will be able to selectively click
on the embedded flash video to enable it.