#135 New option on singleInstance to ignore if already exists [exit code]

[Dyorgio]

Hi guys, I'm posting here a patch that I made implementation.
The idea is to have a new flag in SingleInstance config, instanceAlreadyExistsErr, default true.
If user wants to not return 2 but 0 instead when ERROR_ALREADY_EXISTS is detected, then he sets it to false.
I optimize INSTANCE_WINDOW_TITLE handling too. ;)

[Dyorgio]

Now closing log files before exiting :P.