allow to verify ssl connections against a CA.

This adds three new configuration variables to configure
SSL verification against a CA.

server.ssl_verify: true if it should verify and disconnect
                   if it fails

server.ssl_ca_file: a CA file, is set to the systems CA
                    bundle by default (distri. dependent)

server.ssl_ca_path: alternativly path to a directory with
                    CA PEM files

I tested it and this seems no longer an issue with >= 1.9.3
https://www.braintreepayments.com/braintrust/sslsocket-verify_mode-doesnt-verify
always print FATAL and ERROR logmessages to STDERR

This adds a print to $stderr for log messages with the
level ERROR or FATAL. I did this because in the past errors
connecting to the server weren't immediately obvious but
rather hidden in the logfile. I think it should be right
in-your-face when the connection doesn't work.

It would probaply be better to extend Logger to print
to STDERR but this apparently requires monkeypatching ruby's
Logger. Anyhow I think my solution works for now.
Merge pull request #20 from 4poc/ssl-verify

Implements SSL verification against a configured CA