m4v mime type

Hi Hans,

This adds a mime type for the m4v file format (http://en.wikipedia.org/wiki/M4V). Looks like it is not an official standard, but seeing as both Apache and nginx currently recognize m4v as video/x-m4v, it seems safe to do the same.

For what it's worth, it is working for some mocl videos e.g. https://wukix.com/dist/contacts_scrolling.m4v