patch mail gem v2.5.4 for ruby-1.8.7
patch mail v2.5.4 Exim delivery method
set excon defaults globally
speed up specs
pass a StringIO object to #upload_part

This allows Excon to write to the socket in chunks.
This solves an issue with ruby-1.8.7 where the server consistently
returns a RequestTimeout error, even with the minimum 5 MiB chunk size.
Notifier::HttpPost

Adds a generic HTTP POST notifier.

closes #371, closes #406
added a notifier for Nagios passive checks
added Nagios to list of notifiers in DSL constants
incorporating meskyanichi's suggested changes into Nagios notifier
WIP: bringing spec for Nagios notifier in line with the others...
added template for Nagios notifier
include model's trigger name in the default Nagios check service name
add nagios to expected notifier options for cli specs
Include backup duration in the Nagios check message.
Model#duration isn't actually in seconds... durr
add Config.hostname
cleanup; update specs
Notifier::Nagios

closes #453