Add SSL Features

Greetings,

My current project required the functionality of setting a private CA and client certificate based authentication.  This pull request contains those changes.  I did not touch test.js since I was not sure of the proper way to test a private CA or client authentication with certificates.  Please review and see if you're interested in taking these changes or if you can suggest any changes that need to be made for you to take this pull request.  Finally, thanks for a great module!