thyme.cabal: use text (in platform) instead of utf8-string

∙ Slower than Data.ByteString.Builder though.
Data.Thyme.Format: readsTime and instance Read {{,TimeOf}Day,{Local,Zoned,UTC}Time}
Data.Thyme.Time: compatibility notes and {un,mk}UTCTime, {get,mk}ModJulianDate
Data.Thyme.Format: fix 0-padding of negative years
lens-3.8: Simple Lens ↦ Lens'
Data.Thyme{,.Time}: write some documentation
thyme.cabal: bump version to 0.2.1.1