MailService: SMTP connections are restricted to SSL encryption

[fquadt]

At the moment the MailService only allows for SSL encrypted connections to the SMTP server. The used library simplemail does indeed support three transport strategies (PLAIN, SSL (default) and TLS).
It would be nice to make the transport strategy user configurable by a property.

The only code changes necessary are the new property and a modified constructor call of the Mailer class in MailService.java:
public Mailer(final String host, final int port, final String username, final String password, final TransportStrategy transportStrategy) {

Maybe, in the course of this ticket, it is a good idea to update the simplemail library (currently 1.8) to the newest version 4 or 5 since there have been some new TLS versions and SSL vulnerabilities...