(SERVER-39) Allow a certificate to be passed into a request via X header

This commit adds the ability for a certificate to be supplied in a web
request as the payload of a custom HTTP header, X-Client-Cert.  The
header will only be parsed for a value if the `allow-header-cert-info`
master setting is set to true.  When the certificate is retrieved, it
will be supplied as the client certificate to the Ruby request handler
stack, as a direct replacement for a certificate object that would be
typically be provided over SSL.
I don't think we should ever allow mix-and-match, personally.  I kind of think that if you've turned on the headers, then we should only use what you have in them and that's it.  We should completely ignore any client cert that was presented via https; that seems like it is what you've asked us to do if you turn the flag on.  And it removes any ambiguity and makes it really easy to document without having to include "nested if statements" in our documentation.