(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.
My last comment fell off after a commit, so here it is again:

Is this head name going to be unconfigurable? If not,  maybe we should make a `def` out of it, since it appears a number of times in the code?