Master mono

Added an option for two channel mono output. 

Mixxx has already a mono down-mix when using only one sound card channel.  
But it you need to feed a stereo amp with the mono signal you need a special cable. 

With this patch, you can switch beween Mono and Stereo without tweaking the hardware. 

This feature is required for outdoor gigs, where a Stereo effect is not possible due to the speaker positions.
why is this function so different from the one above that takes an int?  Functions that are overloaded should act similarly.  Anyway you are comparing a double to an int, there should probably be a cast or something to prevent compiler warnings.