Start transport only if there are messages in the queue
merged branch nurikabe/master (PR #323)

This PR was merged into the master branch.

Discussion
----------

Start transport only if there are messages in the queue

When firing the console `swiftmailer:spool:send` command via cron, the file based spool would first open an SMTP connection before trying to determine if there were any messages to send.  This can get expensive when using SMTP services like SendGrid that charge per connection in addition to messages sent.

Commits
-------

40acc60 Start transport only if there are messages in the queue
updated changelog