A number of refactorings to improve correctness and capabilities.

In particular this refactoring includes:

  - Complete redesign of the DateTime.hs file to make better use of
    the data structures of the time package;

  - Adding support for a number of additional XSD data types;

  - Adding a test suite, mainly focused on the datetime XSD data
    types.

  - Replace most uses of String by Text from the text library.