Shortened screen notification dividers
Callbacks now have access to screen notification

```
Paratrooper::Deploy.new('amazing-app') do |deploy|
  deploy.add_callback(:before_setup) do |output|
    output.display("Disabling newrelic ping")
    # code to disable new relic
    output.display("Yep, done did that!")
  end
end
```
Change migration_check code style to match others
Fixed migration check not being available

* Now checking for last deployed commit in setup before anything else
* runs
Making #callback and #notify private
More method documentation

[skip-ci]
Merge pull request #49 from mattpolito/custom_callback_screen_notifier

Add screen notification when custom callbacks are added