Added time_adjust_duration to LoglineDecoder config options so users can deal w/
log files that aren't in UTC but don't have the time zone embedded in the timestamps.
Added test for time_adjust_duration, use gs.Equals instead of == and gs.IsTrue for
better test failure output.
Switch from 'time_adjust_duration' to 'timestamp_location' for the config setting to
do on-the-fly timezone adjustments for parsed timestamps.
Use lower-case and underscores for idiomatic TOML config options.
MessageFields -> message_fields, and update the docs.
Merge pull request #294 from mozilla-services/features/ll-timezone-adjust

Features/ll timezone adjust