First pass at converting TransformFilter into LoglineDecoder.
Move generally useful MessageTemplate type and InterpolateString function to
decoders.py for better visibility.
Fixup LoglineInput test for new decoder usage.
Added initial LoglineDecoder test.
Merge branch 'dev' into features/logline-decoder
Add test for SeverityMap functionality, and make DecoderRunner available to
decoders that want it.
Merge pull request #170 from mozilla-services/features/logline-decoder

Features/logline decoder

r+ benbangert