Install pah in a existing rails app

Now it's possible to install the pah setup in a existing rails app. So the following:

```sh
pah new my_app
```
is the same as:
```sh
rails new my_app
cd my_app
pah install
```

To achieve this, a minor change was made. When creating a new app, because the nature of thor tasks, instead of `pah my_app` you need to use `pah new my_app`. But, I think that it's not a problem, given the benefits of thor.