Mplayer fixes

Fix for Issue #11, plus a few other minor improvements.

The problem was that MPlayer resumes playback when issued the `get_time_pos`
command (inside `playback_status`).

I decided to use `pausing_keep_force` instead of `pausing_keep`, it was
introduced in 2008 (r27665) and while it is still marked as experimental, it
should be safe for getting and setting simple properties. The disadvantage of
using `pausing_keep` instead of `pausing_keep_force` is that MPlayer advances
one frame every time a command is called. Since `playback_status` is called
every 0.5 seconds, orochi will quickly move through a song even though MPlayer
is "paused".

Do you think we should check the current version of MPlayer to see if it
supports `pausing_keep_force`? One alternative, according to this thread [1], is
to simply call `pausing_keep_force` first, and if MPlayer does not respond
assume it does not support it, and fallback to `pausing_keep`.

[1] http://mplayerhq.hu/pipermail/mplayer-users/2008-September/074473.html