(SERVER-18) Add x-client-dn and x-client-verify support

Not sure if this is the scope we want to put this setting at.  I had been thinking this was more of a Puppet master-specific thing, although we don't really have a "master" specific section defined yet and it probably doesn't make sense to put this in the jruby-puppet section.

I suppose other services could make use of this at some point.  If that were to happen, though, I'm thinking it might make sense to pull the validation of the headers -- including the setting -- into the webserver service so its closer to where the data comes off the wire.  For now, though, I'd be inclined to create a new master section and put this setting in it.  Curious what @cprice404 and others think...