MPD binding improvements

Adds support for track names, artist names and ability to read and set volume.

Example items:

```
String CurrentTrack    "Current track [%s]" { mpd="TITLE:bad:tracktitle" }
String CurrentArtist    "Current artist [%s]" { mpd="ARTIST:bad:trackartist" }
String ConcatInfo       "Now playing [%s]"

Switch Mpd_Bathroom_StartStop       "Start/Stop"   (Bathroom)  { mpd="ON:bad:play, OFF:bad:stop" }
Switch Mpd_Bathroom_NextPrev        "Track control"   (Bathroom)    { mpd="ON:bad:next, OFF:bad:prev" } 
Dimmer Mpd_Bathroom_VolumeControl   "Volume [%d%%]"       (Bathroom)  { mpd="INCREASE:bad:volume_increase, DECREASE:bad:volume_decrease, PERCENT:bad:volume" }
```

Sample rules (to concatenate artist and title):

```
rule "concat"
when
  Item CurrentTrack received update or
  Item CurrentArtist received update
the
  ConcatInfo.postUpdate(CurrentTrack.state.toString + " / " + CurrentArtist.state.toString)
end
```

Example sitemap:

```
Text item=CurrentTrack
Text item=CurrentArtist
Text item=ConcatInfo
Switch item=Mpd_Bathroom_StartStop mappings=[OFF="Pause", ON="Play"]
Switch item=Mpd_Bathroom_NextPrev  mappings=[OFF="Previous", ON="Next"]
Slider item=Mpd_Bathroom_VolumeControl
```