Allow sending of SIGUSR2 to initiate a reload of all log files

Hello,

I have had a go at adding the ability for a SIGUSR2 signal to be received by the pm2 process which will then initiate all of the child processes to reload their log files. This is in an attempt to solve the logrotate issue mentioned in #114 and #370 without the need to use copytruncate.

If you are happy with the implementation in principle then I will have a go at adding some tests