Ability to run a never ending process and also kill it

This PR adds the functionality to kill a process.

By adding `kill: true` to options, you can setup a process that never ends (like a server) and run the task again with `:kill` arg to kill it.

I added this because I need to run webdriver against my application. Rather than spinning the app up myself, it is preferable that grunt handle the job. To test that the functionality is sound, run `grunt killTask`. 


```javascript
            neverEndingTask: {
                command: 'node tests/server/server.js',
                options: {
                    async: true,
                    kill: true
                }
            },

    grunt.registerTask('killTask', ['shell:neverEndingTask', 'shell:curl', 'shell:neverEndingTask:kill']);
```