Add shutdown hook to clean up child processes

Hello,

It is possible on Windows 7 machines to have an orphaned phantomjs process running.
In example : 
1) Create a project that uses jasmine-maven-plugin with PhantomJSDriver
2) Run mvn clean install
3) At the moment that phantomjs is launched, do a ctrl-c to kill maven

The phantomjs process is not terminated with the maven process.   phantomjs must be killed manually

This is not a problem on linux machines.  Only Windows.

The shutdown hook is meant to clean up any orphaned child processes.

best regards,
Mark