Support for US/Euro time formats. CLI/Library supported.

After noticing the invalid date error, I figured it was probably due to the various Nagios time formats supported and OOB configuration. I've added support for US/Euro, but not for iso8601 or strict-iso8601. Defaults to 'us' because at least on CentOS, that's the default for me. *shrug*

Figure someone else may want this support to at least start using nagiosharder if they have the same problem.

Awesome gem, nice work overall!