allow to filter optionally by hostgroup

[j1n6]

First, thanks for creating this awesome gem!

I am able to get most information from gem APIs, but in this case I would like to get detailed Nagios checks filtered by hostgroup.

This PR adds additional parameter hostgroup to site.service_status to filter status details by hostgroup. I've tested on Nagios v3 and v4, they do have this available in status detail view.

To be more abstract, I think this method might be better renamed to status(options), but renaming would cause API breakage for gem users - so i kept the same method name.

[j1n6]

feedbacks are welcomed.

[vStone]

You should probably not include the update version commit to let the owner decide when to release a new version ;)

But i would welcome this feature.