Reporter -> TextReporter, AggregatedReporter

With this change it will be possible to create reporters which do various stuff with parsing results (I already have CertDBReporter ready, which stores certificate descriptions in CertDB). This change mimics previous reporter, so all AsyncResults are stored till report is called. I have follow up commit ready which fixes that, and allows reporters to work in more streaming manner (but I think it should go in separate pull request). I also added little change here - I add certificate descriptions from scan callback, so it's ready for CertDBReporter.
Could add single-line docstrings... :wink: