Comments and cleanup

- lengthy comment about schema interpretation for insert and search
- QuickCheck dependency removed, which was added by mistake with
  commit 38d86c638bf0138351c3a858e5700465011f6c1b
Add Date to schema types enum

- rename Normalizer enums to not clash with type names
- minor json adjustment (remove "c" prefix of types)
- NOTE: date-normalizer is undefined
Add date range and change insert analyzer behaviour and range parsing

- add date parser/validator code from
  hxt-xmlschema/src/Text/XML/HXT/XMLSchema/W3CDataTypeCheck.hs
- see TODO and XXX
Add validator and parser for any of the date formats
Make normalizeDate safe (do not normalize invalid dates)