Added support for triggers on various events

This patch add support for triggers on various events like
add/release/expire a lease from dhcp server.

Add three parameters to dhcp::pools:

oncommit: An array of triggers to run on commit (start a lease) (one element for each action)
onrelease: Array of triggers to run on releasing a lease
onexpiry: Array of triggers to run on expiring a lease
Merge pull request #1 from hkumarmk/add_event_support

Added support for triggers on various events