Fix syslog support

Hi there, thanks for the recent rework in the syslog support.  This is a set of changes which make the syslog output a bit better, eg:

    Sep 16 23:03:38.496672 somehost supervisord[11630]: supervisord started with pid 11630
    Sep 16 23:03:39.501762 somehost supervisord[11630]: spawned: 'some-proc' with pid 11637
    Sep 16 23:03:39.507778 somehost some-proc: stdout: /opt/someone/some-proc/someproc/app.py
    Sep 16 23:03:39.511096 somehost some-proc: stdout: Activating virtualenv
    Sep 16 23:03:39.516475 somehost some-proc: stdout: Sourcing /etc/envs/some-proc.env
    Sep 16 23:03:39.520331 somehost some-proc: stdout: Launching /opt/someone/some-proc/someproc/app.py
    Sep 16 23:03:39.868059 somehost some-proc: stderr:  * Running on http://0.0.0.0:8000/
    Sep 16 23:03:39.868579 somehost some-proc: stderr:  * Restarting with reloader

Along the lines of the new "stdout_syslog" and "stderr_syslog" boolean values, I have created a new "syslog" top-level configuration.  This has additional effects: if the defined processes have no logfile defined, then it will instead forward to syslog.

Closes issue #162