Feature: Add SSL certificate to logger

The tcp input for logstash allows the data to be transmitted securely
using an SSL certificate. This commit adds a fourth (optional) argument
to the logger to specify the path of an ssl certificate. If this is set
then it will use a secure socket.

Most of the code in this commit is heavily inspired by https://github.com/elasticsearch/logstash-forwarder/blob/master/lib/lumberjack/client.rb#L76