Add native support for 'refresh' attribute

Prevents converting 'refresh' value to a string which results in:

```{"timestamp":"2013-08-01T20:48:15.424623+0000","level":"info","message":"/opt/sensu/embedded/lib/ruby/gems/2.0.0/gems/sensu-plugin-0.1.7/lib/sensu-handler.rb:75:in `filter_repeated': undefined method `fdiv' for \"600\":String (NoMethodError)"}```