Readd method name check to the metrics worker

In order to assure the score details attribute is only fetched if it is
available, I have readded the respond_to? method.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add status and logger calls to RoI metric

In order to give feedback for the preprocessing of the metadata, which
can be quite time consuming, in the Richness of Information metric, I
have addded a reference to the worker.

This way the status method at can be invoked. In addition I have added
another logger call as well.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix empty string problem in Accessibility metric

The same metadata record with a whitespace string has caused problems in
the Accessibility metric. I have moved the skip? method into the general
metric class and added similar condition checks.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>