[Reviewer: Rob] Use UTC in log files and filenames

Rob,

Please can you review my fix to use UTC in log files and filenames (fixes #92)?

This basically consists of adding `--utc` to all `date` commands, but I've also added `%Z` to the date format specifier when logging to ensure that it specifies the timezone explicitly.  Note that I haven't specified the timezone explicitly in filenames - I think it makes them longer without adding much value (and there's precedent for this elsewhere).

I've live-tested on a sprout node by generating a diagnostics dump and checking its filename and contents.  I haven't tested on any other node types, but can't see any reason for any difference.

Thanks,

Matt