Add Spotify control via GUI

_player_engine.php
* Formatting cleanup, no functional changes

command/index.php
* Add code to redirect playback commands to Spop when Spop is playing

db/index.php
* Modify spop commands to use new 'sendSpopCommand()' function

inc/player_lib.php
* 'sendSpopCommand()' now returns the response
* add function to get Spop state
* some formatting cleanup

js/volumio.api.js
* Add global 'GUI.SpopState' object for Spop status
* rename MPD status field from 'GUI.json' to 'GUI.MpdState' to prevent confusion
* modify 'renderUI()' to handle displaying status of two different daemons
* new 'backendRequestSpop()' function to call '_player_engine_spop.php'

js/volumio.playback.js
* Use the renamed 'GUI.MpdState'

templates/indextpl.html
* Remove the "Spotify Stop" option
Merge remote-tracking branch 'upstream/master'
Merge pull request #44 from ning-yu/master

Spotify controls added