Add Basic Auth support

This adds basic auth support. Requests to the metrics endpoint will require basic auth if -auth.pass is set.
It seems like an oversight to me that "log" is still used at all in the node exporter, while most of it is using "glog". I think it should all use "glog" and then we don't need to import "log". But it would be nicer to have that in a separate change (at least separate commit).