Fix --ct_server arguments to cpp/ct  (now requires a URL)

Also:
* display OpenSSL on startup of test_running_server
* When DER-formatted proof can't be converted to PEM, write a
  message to stderr (but keep on failing)
Merge pull request #176 from trawick/fix-sslconnect-log-url

Fix --ct_server arguments to cpp/ct  (now requires a URL)
Don't use deprecated function.
Fix signed/unsigned warnings.
Switch from google-test to google-mock.