Log in separate files

## The goal :
In order to process the log afterward, we need to separate the logs in the different files. Thus, there is no mix between the log (for the stacktrace).

## How is it done :
There is a new option `-s or --separate_log_file` that permits to log in different file. Each file is name with the following pattern : appname-out-ID.log and appname-err-ID.log.

For instance : 

`pm2 start echo.js -i2 -s` outputs
/Users/anthony/.pm2/logs/echo-err-1.log
/Users/anthony/.pm2/logs/echo-err-0.log

The pull request also includes the possibility to reopen the file with a signal. It's very useful for tools like `logrotate` in the `postrotate` command.

please give feedback and discuss.