Added safe_mode property

In reference to issue #79, I found that lib/puppet/type/sensu_check.rb was lacking the definition for safe_mode.

Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Invalid parameter safe_mode at /etc/puppet/modules/common/sensu/manifests/client.pp:32 on node [removed].

I'm a bit new to this, but I think this will correct that error; it did in my environment.