(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.
Do you think it's safe to assume that this is the correct behavior?  It seems like we're breaking new ground here, because in the old master the user would have to figure out some way to set the environment variable, right?  And it would be up to them to decide what cert to use in the case of a chain?

Or did we have some passenger config that was setting the env var, and if so, do you know what it would do in the case of a chain?