Option to use SSL for PushoverHandler (enabled by default), and close connection after sending a message to PushOver.

Solves #146: When sending messages in rapid succession to pushover.net only the first one is sent.
The SSL option is needed when sending messages to users that are not the pushover.net app owner; pushover.net doesn't accept those messages over plain HTTP.
Merge remote-tracking branch 'AlexButter/master'
Fix permissions
Fix test suite