Initial checkin of Pushover.pm

Add support for notifications using the Pushover.net service.

 - Support notify() command to push a message
 - Implements basic rate limiting of duplicate messages
 - Tracks receipts and acknowledgment of priority 2 messages
Add global disable parameter

Pushover_disable = 1 to disable all notifications.  Messages will still
be logged as if notifications actually occurred.   Can be overridden on
individual notify() calls.
Fix Pushover compatibility with perl 5.8.8

Also, validate the incoming Hashref for new() and notify() so that
a malformed hash doesn't cause a crash.   if new() is malformed,
disable Pushover.    If notify() is malformed, allow the
notify to go forward but without any optional parameters.
Merge pull request #373 from hollie/add_pushover_support

Adding support of Pushover.pm