Added datatypes.py
Added PythonVersion
Updated PythonVersion

* Added docstring
* Corrected __init__
* Formatted using (auto)pep8 and pyflakes
Use `platform.python_version` by default
Renamed class
Added __hash__
Revised for pep8
Added IronPythonValidationError
Added validate_ipyexe
Now search_ipy_env use validate_ipyexe
Recommit
Added major_minor
By default use 'x.y' for backward compatibility
search_ipy_reg now validates exes
When no IronPython executable was found
foundvers is list, not dict
Updated docstring
Better error handling
Revised error message
Updated search_ipy