Initial adding of X.509 client certs
Adding documentation
Clarifying docs that client cert also should have key
Last time, getting the wording right.
Fixing the way we load the cert and key.

Turns out, you have to read the cert into Net::Http.cert.
Helps if I also document the environment vars...
Merge pull request #2612 from snackbandit/509

Initial support of X.509 client certificates