SSL Support

This patch adds SSL support in two different fashions:

1. No SSL verification: connect and transmit encrypted with no verification check from the host that this sender is authorized. In this case, the user would only specify `ssl_enable: true`.
2. SSL Verification: Challenged during connection that the public certificate is acceptable from logstash's configured CA Cert(s). In this case, the user would enable SSL and also specify the client's certificate and private key to establish the SSL connection. The certificate would need to be installed on the logstash server to verify the sender as trusted.

I've also added some general configuration documentation under the `/docs` directory which is created to hold any future documentation.

Since the test certificates and keys are self-signed, I had to update the travis configuration to set the `NODE_TLS_REJECT_UNAUTHORIZED=0`, as node by default will reject self-signed certificates. This also means this environment variable needs to be set prior to running `npm test` locally, which I've updated in the readme.