* Add ability to get Freeswitch application data via snmp

On your Freeswitch server install the script from
https://github.com/librenms/librenms-agent/blob/master/agent-local/freeswitch
into /etc/snmp, make it executable and adjust it to your installation.
Configure your snmpd to use the sript as an extension. For example on Debian
add the following line to your /etc/snmp/snmpd.conf:

extend freeswitch /etc/snmp/freeswitch

Activate the Freeswitch application for your server on your LibreNMS server.
it should now start to receive data from your Freeswitch server via snmp.

* Graph "Calls" and "Channels" of Freeswitch agent

The Freeswitch agent sends Calls and Channels count and they weren't graphed
up to now.  This is especially useful in settings where the Freeswitch
server does not act as a Gateway and thus the agent does not sent
Incoming and Outgoing Call numbers.

* Cleanup commented code and add myself to authors