Add support for "Epoch" time_format settings to the PayloadRegex
and PayloadXml decoder plugins.
Simplified the multiplier / divisor calculation to have less special casing,
removed reference to Timestamp and TimestampLayout in
ScribbleDecoder comment.
Add a test for the new precision validation.
Ignore extra precision instead of raising an error.
Merge pull request #1150 from mozilla-services/epoch_time_parse

Add support for "Epoch" time_format settings to the PayloadRegex