NMS-6638: Initial revision for AvailabilityMonitor's documentation

http://issues.opennms.org/browse/NMS-6638

Cyrille
NMS-6638: Added info about remote poller + implemented some improvement requested in https://github.com/OpenNMS/opennms/pull/62

Cyrille
NMS-6638: Changed service name + updated the doc with latest info from jeffg (NMS-6735) regarding remote polling.

Cyrille
Merge pull request #62 from StCyr/cyr-doc-AvailabilityMonitor

NMS-6638: Initial revision for AvailabilityMonitor's documentation