Merge pull request #23 from mindstorms6/master

Adding playMixID and playURL
Formatting & documentation of new play feature
Show help when using play command without arguments
Copyright notice mentions contributors
Use pausing_keep_force to stop MPlayer resuming playback while paused

`pausing_keep_force` was introduced in 2008 (r27665), and while still
marked as experimental, should be stable for getting and setting simple
properties.

`pausing_keep` causes MPlayer to advance one frame every time a command
is issued.

Fixes issue #11.
Check for MPlayer support of pausing_keep_force (r27665)

`pausing_keep_force` was introduced in r27665, and was released in
version 1.0rc3.
Pre-compile regular expression, streamline match checking
Ensure MPlayer never shows a video output

Be overly paranoid with MPlayer arguments.
Move call to MPlayer.terminate before raising exception
Add note in README about MPlayer minimum version
Merge pull request #20 from quigybo/mplayer_fixes

Mplayer fixes
Renamed skip to next_song, added short aliases (fixes #22)
Handle SIGINT (fixes #1)
README formatting
Use `pausing_keep` in stop command (refs #20)
Improved status output
α -> β