StartTLS Support

This PR is largely inspired by @MitchelSBlake's changes in #50, albeit with no visible changes to the interface.

It appears that port 465 is generally used for strict SSL/TLS connections, while port 587 is used for StartTLS. See the following article from FastMail on this:

https://www.fastmail.fm/help/technical/ssltlsstarttls.html

This means we can keep the same interface controls while adding support for StartTLS by checking the port number that the user selected. Also, there appears to be precedent for this in other apps, such as Carbon Copy Cloner:

![screen shot 2014-07-20 at 4 32 06 pm](https://cloud.githubusercontent.com/assets/962647/3638891/46624c6c-1067-11e4-8c38-0a3b15cc484f.png)