chain vs single ca cert

updated client and server to support chain and leaf cert responses.

# create a new CA
scepserver ca -init
# start server
scepserver -depot depot -port 2016 -challenge=secret

# in a separate terminal window, run a client
# note, if the client.key doesn't exist, the client will create a new rsa private key. Must be in PEM format.
scepclient -private-key client.key -server-url=http://scep.groob.io:2016 -challenge=secret