(Fixes #69) Remove --ssl-verify and provide peer_cert

The get_peer_cert method of EM doesn't return anything unless the cert
has been verified. The --ssl-verify option of thin actually doesn't do
anything. These two behaviors combined mean that
env['rack.peer_cert'], which was introduced in thin 1.2.8, always
returns nil. Since --ssl-verify never actually caused a verification to
happen, it is better to remove that option until a fully verification
process is put in place. However, the peer_cert can be made available in
--ssl mode by always "verifying" the cert, thereby providing the client
supplied certificate, if there is one, available in
env['rack.peer_cert'].