Updated calendar documentation, including ISO

- Validated docs for ISO calendar
- Corrected small error and added exception example in replace
- Removed gregorian.week_number again (will be better implemented with nday_before or after)
- Two small other fixes
Added tests for ISO calendar
Added tests for to_rata_die
Made __ne__ definition independent from __eq__
Added the ISO calendar code

- added the iso.py file
- fixed small errors in the test code