Add a time limit option for the SMTP connection

[MatthiasKuehneEllerhold]

As described in the documentation, this PR will add the option for a time limit for the SMTP connection.

If you like the general direction, I'm willing to add unit tests tomorrow.

[Ocramius]

This patch requires extended testing (probably via a clock object)

[MatthiasKuehneEllerhold]

I've added a test with the help of Reflections.

There may be one issue: this PR sets the current connection to null if the time limit was reached. This means we're relying on lazyLoadConnection() to recreate the connection. But we cant recreate the connection if it was injected through setConnection(). So (maybe / probably) we're getting a different connection if the time limit is up and the connection is recreated using lazyLoadConnection() if it was injected through setConnection() on the first mail.

Example: We're injected a mocked connection in the Unit-Tests and do not rely on lazyLoadConnection().

EDIT: Is targetting the master branch correct?

Possible solution: Implement the auto-reconnect in the Zend\Mail\Protocol\Smtp class instead.

[weierophinney]

Thanks, @MatthiasKuehneEllerhold!