(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.
So, this logic states that a provided cert from a proper HTTPS exchange would possible override anything passed in from the HTTP headers. This seems sane, but we should probably ensure that this is clearly documented behavior. And as @cprice404 stated below, we should probably figure out (maybe in The Future™) what to do in the case of a real cert/head DN mismatch.