(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.
Yuck, I didn't realize that we already had that somewhere.  :(

I agree with everything @nwolfe said but don't spend too much time on it right now, we have bigger fish to fry.  If there's stuff you agree you'd like to change but seems like it will take a non-trivial amount of time, let's just open a new ticket so we can go ahead and get this stuff merged.