FEATURE: Initial implementation of Volume Control

Provides new options in Android app and in Server.
If Volume control is enabled in android app, then server will
run specified commands when it receives a volume key event.
These commands are configured via the server tray-icon menu.

This has been tested on Android 4.4 controlling Linux Mint.

On linux, I use the following commands to control volume when the signal is received:

    amixer set Master "5+"  # Volume up
    amixer set Master "5-"  # Volume down