Import webmquicktime webm writer

Initial import of the libmkv directory from the webmquicktime[1]
project, at commit fedbda1.

[1]: git://review.webmproject.org/webmquicktime.git
     commit fedbda18de899ff94855cb334de7e471036fbf1d

Change-Id: I1564a0ebfa72293fc296ee02178196530dfd90e4