Systemd support and nagios_init deprecation

pynag.Control.daemon
==================

    Systemd based systems now function correctly and daemon assumes use of
    service_name instead of nagios_init. The main rationale for the change is
    how to query if the nagios service is running. "service nagios status" is pretty
    expensive so what daemon does is lookup pid file if nagios_init or
    service is used but "systemctl is-active nagios" on systemd based
    systems.
    
    Added a bunch of tests for the new and old functionality.
    
    Switch was also made from internal subprocess.Popen calls to use the
    pynag.Utils.runCommand which is easier to mock from a testing
    standpoint.
    
    sudo use in nagios_init is also deprecated and is a flag now, sudo=True
    when creating a new daemon() instance.
    
    All calls now populate self.stderr and self.stdout which calling
    programs can utilize to determine why something broke.

    New methods, start and stop.

The prefered way to use daemon after this change is without nagios_init
```
>>> from pynag.Control import daemon

>>> d = daemon(
...  service_name="icinga",
...  nagios_bin="/usr/sbin/icinga",
...  nagios_cfg="/etc/icinga/icinga.cfg")

>>> d.verify_config()
True
>>> d.stdout
'Output from command'
```