Introduce possibility to create afm execution straight from md Obj #572

Currently such ugly thing is necessary:

final Execution execution = new Execution(new Afm()
                .addAttribute(new AttributeItem(new IdentifierObjQualifier(attr.getDefaultDisplayForm().getIdentifier()), "a1"))
                .addMeasure(new MeasureItem(new SimpleMeasureDefinition(new UriObjQualifier(metric.getUri())), "m1"))
        );

Let's introduce some possibility for easier Execution creation. Possibilities to consider:

• AttributeItem and MeasureDefinition constructors having Attribute / DisplayForm / Metric as arguments
• add convenient addAttribute and addMeasure methods
• introduce some factories or builders?