libmpv trivia

1. I used "0.0.0" as version in the pc file, but this is most probably wrong (I just can't figure out where to get mpv's version inside waf).
2. It may be just me, but `mpv/client.h` looks nicer than `libmpv/client.h` (if you don't agree, just ignore the second commit). Since there hasn't been a release with libmpv, this shouldn't cause any trouble.

Speaking of releases, is there an ETA or an action plan for 0.4.0?