Initial spike for pleaserun support

I experimented with [pleaserun](https://github.com/jordansissel/pleaserun) support for fpm-cookery.

Example:

```ruby
class Redis < FPM::Cookery::Recipe
  # ...

  # Creates first startup script for redis-server binary with default settings
  startup_script '/usr/bin/redis-server'

  # Creates a second one
  startup_script '/usr/bin/redis-server',
                 :name => 'redis2',
                 :args => '--port 1234',
                 :description => 'Second Redis instance',
                 :user => 'nobody',
                 :group => 'nobody'

  # startup_script options: :name, :args, :user, :group, :description, :chdir, :umask, :prestart
  # ...
end
```

Looking for feedback on the API! :) /cc @beddari @jordansissel