Add abstract delegate converter class

[imagejan]

This PR closes #182 and adds AbstractDelegateConverter that allows to specify new converters that chain together two subsequent conversions like this:

@Plugin(type = Converter.class)
public class StringToDatasetConverter extends
	AbstractDelegateConverter<String, File, Dataset>
{

	@Override
	public Class<Dataset> getOutputType() {
		return Dataset.class;
	}

	@Override
	public Class<String> getInputType() {
		return String.class;
	}

	@Override
	protected Class<File> getDelegateType() {
		return File.class;
	}
}

I am not sure about how to implement the test: is it possible to create private nested classes that can be discovered as Plugins during test time?

I'd appreciate some advice here if you have time, @ctrueden.

EDIT: I think I figured it out. I force-pushed the latest commit, adding a test for the abstract class. This should now be ready for review.

Test coverage of the added classes: 100% 🙂

[ctrueden]

Cool, thanks a lot @imagejan!