Major refactoring

Please find enclosed a major refactoring of the xsd package. The changes include

  - A complete redesign of the DateTime.hs file to make better use of the data structures of the time package;

  - Support for a number of additional XSD data types;

  - A test suite, mainly focused on the datetime XSD data types.

  - Replace most uses of String by Text from the text library.

  - To improve performance a bit use Attoparsec rather than Parsec.

Note, these changes make the external API incompatible with the previous xsd package so I suggest increasing the version number to, e.g., 0.4.0.