Add task generator.

ref #1200 

```
$ padrino g task
Usage:
  padrino-gen task [name]

Options:
  -r, [--root=ROOT]                # The root destination
                                   # Default: .
  -d, [--description=DESCRIPTION]  # The description of your application task
  -n, [--namespace=NAMESPACE]      # The namespace of your application task

Description:

	padrino-gen task generates a new task file.

```

How about this one?