Extend @As with a provider mechanism.

[Airblader]

This change extends the @as annotation for step / stage methods and classes
with an AsProvider provider() method to allow a more flexible way of
customizing the result.

fixes #189

[Airblader]

Obviously missing things:

• Documentation
• Make it more like CaseDescriptionGenerator with the special NO_VALUE value to (somehow) expose the nameWithSpaces functionality (and update the documentation).
• Tests

@janschaefer I liked having two separate as() methods for the method versus class cases better. In fact, we could we even differentiate between calling this for a step method and for a test method, since the annotation works on both. Should we?

[Airblader]

@janschaefer OK, you can take a look at it now.

[Airblader]

So I think one can make either argument for or against having this in the DefaultAsProvider. My rationale is that this way, a user can extend from this provider and utilize this, e.g., in the spirit of the original motivation for #189:

@As( provider = ReplaceTodayProvider.class )
public SELF a_request_with_request_date_TODAY() { … }

public static class ReplaceTodayProvider extends DefaultAsProvider {
    @Override
    public String as( As annotation, Method method ) {
        String words = super.as( annotation, method );
        return String.replace( "TODAY", formatDate( new Date() ) );
    }
}

[Airblader]

@janschaefer Updated.