(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 need to do any DN validation right now as long as we eliminate the mix and match behavior.  Either you've told us you want to use headers and we ONLY use headers, or you've told us that you don't want to use headers and then we pay attention to the real HTTPS client cert.  Not both.