plugins.dailymotion: initial VOD support

This adds support for Dailymotion VODs, using `SegmentedStream` (based on the UStream plugin). It works well in the cases I've tested, (VLC + file output which is seekable), even though I get a lot of
`[flv @ 0x7f2dccc01c20] Skiping duplicate index`
errors when playing the file. I'm not sure if this is a big issue.