Implementation of CUI Date class

[TonyFlury]

This is the implementation of the CUI Date class - includes :

1. New module : cui.py - includes the CUI Date class and templates for NHSNumber and Name classes.
2. New module : tests/test_cui.py - tests the CUI Date class 21 test cases.
3. Documentation : updated README, AUTHORS, HISTORY & docs/usage.rst
4. Documentation : created docs/CUI/Date_Time.rst

Tests executed :

• tox tests against py26, py27, py33, py34, py35
• coverage tests using py27 as a base - 89% test coverage - templates for NHSNumber and Name classes not tested.
• tested setup.py - builds py2.py3 wheel without errors.