RF: Refactor date detector and date template elements

Changes include to use Python class properties, merge some date
patterns, and change ISO8601 date template to DatePatternRegex class.
ENH: compress DateDetector templates more
TST: Fix dates in ISO8601 being converted back to local time.
Merge pull request #598 from kwirk/date-detector-template-rf

RF: Refactor date detector and date template elements