Added ability to execute shell commands on advice

Added simple framework to execute arbitrary command-line commands on any new advice. This can allow for arbitrary functionality to be 'plugged-into' Gekko without having to code it into the application. 

For example, this can be used to invoke a notification service like [Pushbullet](http://www.pushbullet.com/") so that advice can be delivered as push messages to Android devices instead of through mail. This can also be used to replace the built-in mail framework so that Google accounts are not necessary to send emails.