(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.
hmmm... this behavior seems tricky, too.  Have you thought of a use case where someone would want to send headers for the DN but still actually use the real https client cert?  That seems weird to me.  It seems like we'd at least want to check that the DN matched the real client cert, but I actually think I'd be inclined to go farther and intentionally pass `nil` to the ruby layer for the cert in this case.