Major improvements and tunings ***BREAKS BACKWARD COMPATIBILITY***

PHP 5.5 is now required.

May still cotain bugs, this is a work in progress:
- Table->select() method is now a generator. Db::FoundRows() calls
  must be made separately when needed. Code using this method must
  be adapted accordingly.
- Fixed regression in Table->cast()
- New DoPhp:duration() methos returning total execution time
- Model field definition are now internally handled as
  FieldDefinition instances instead of arrays. Template code and
  extensively customized models may need some rearranging.
  Some bugs related to this change may still be present
- Admin (resume) CRUD tables now output datatables Json data.
  Greatly reduced load time and bandwidth usage. Improved code
  reducing memory usage too.