Generating return value automatically where method is not yet implemented

This adds method-body generation in a specific case:

1. A failing example exists where a method is expected to return a scalar
2. There are no other contradictory examples
3. The method is currently empty (aside from comments)

When I write the spec:

```php
class HelloWorldSpec extends ObjectBehaviour
{
    function it_should_return_hello_world()
    {
        $this->greet()->shouldReturn('Hello, World');
    }
}
```
I am asked on subsequent executions:

*  Do you want me to create `HelloWorld` for you?
*  Do you want me to create `HelloWorld::greet()` for you? 
*  Do you want me to make `HelloWorld::greet()` always return "Hello, World" for you? 

And end up with the class:

```php
class HelloWorld
{

    public function greet()
    {
        return 'Hello, World';
    }
}

```