Sending alerts via SNMP

[joachimhs]

When an alert threshold has been breached, an SNMP Trap message needs to be issued for every period the threshold is breached (maybe a single snmp trap might suffice ?).

A single SNMP Trap message needs to be issued when the threshold is no longer breached.

Changes needs to be made to the Alert Administration panel to be able to select an SNMP alert action.

The SMNP functionality should be implemented as an Alert Export Plugin, as specified in issue #13