Implement `mDRunner`, a wrapper around the dRunner default DecoderWrapper
implementation that a MultiDecoder can hand to any subdecoders that require one.
Add a test for sub-DecoderRunner delegation.
Record change.
Merge pull request #447 from mozilla-services/features/multidecoder-runner

Features/multidecoder runner
Use a fixed hostname to prevent variability in tests leading to false failures.
Merge branch 'dev' into issue_444