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
Spotify disconnect bugfix

* Fix database request hanging if spopd is not running
Spotify disconnect bugfixfix

* Put "if ($sock)" check in sendSpopCommand() instead
Merge pull request #45 from ning-yu/master

Spotify disconnect bugfix
Spotify search and add to queue

db/index.php
* Add Spotify results to 'search' query output
* Make callback listeners for Spop add-to-queue commands

inc/player_lib.php
* Add Spotify search function which returns array of tracks matching query

js/volumio.playback.js
* Add callbacks for Spop add-to-queue commands

templates/indextpl.html
* Add context menu items for Spop add-to-queue
Merge pull request #46 from ning-yu/master

Spotify search and add to queue
Different Icons for Spotify Tracks
Styling Update System
Styling Update System
Play Spotify on double click
Fix Double Click action on WebRadio
Direct DSD and DSD Over PCM Support
1.5 MOTD
Force scrollbar to appear

* Prevents the UI from 'jumping' sideways when switching between a view that fits in the window and a view that overflows
1.5 Version in Credits
Adding Jotak to credits
Update Check System
Update credits.html