Add callbacks to default deploy actions

Hi Matt, I wrote some code to add callbacks to default deploy actions.
Now, the user can do:

```ruby
deployment = Paratrooper::Deploy.new("myapp")
deployment.before :push_repo, "echo 'Hello there' > log.txt"
deployment.deploy
```

I think some points could be improved. Currently, to execute a command I'm using `Kernel#``, and we can just use strings as command line.

What do you think about these?

related to #14