Making easier to use PubSub on a Supervisor

[kelvinst]

Closes #5

As you can see, I added the start_link/1 so one can use Supervisor.child_spec(PubSub) on
supervisor's child list.

Also, I've created the child_spec/1 function, which makes it possible to use directly PubSub
on the supervisor child list.

[coveralls]

Coverage remained the same at 100.0% when pulling 9ccf47d on kelvinst:supervisor into 9313ed2 on simonewebdesign:master.

[simonewebdesign]

Thank you!