Optionally use avconv/ffmpeg for .ogv transcodes

Set $wgFFmpeg2theoraLocation = false to use avconv/ffmpeg instead of
ffmpeg2theora for producing .ogv transcodes; beware this disables
creation of Ogg Skeleton metadata tracks.

Note that ffmpeg2theora 0.29 on Ubuntu Trusty and OSX/Homebrew seems
very flaky and produces files with broken audio tracks.

Bug: 67953
Change-Id: Iaf5d51200e618b4b4a03f3cdba31149a2fcf7c71