Make <audio> and <video> elements work

The content module expects to find ffmpegsumo.so in a 'Libraries'
directory inside Brightray Example.framework. The Libraries directory
did indeed exist, but only down inside the Versions/A directory. Now we
create a symlink to it at the top level (and to the Frameworks directory
as well while we're at it), just like standard frameworks created by
Xcode have.

Fixes https://github.com/brightray/libchromiumcontent/issues/16.