Field.to_xml -> write_to_xml

- This is in keeping with MS API names.
- Moved the eprops member variable from Contact to Item
Minor cleanup in Item property initialisation
str() cannot return None; fix Gender.str
Rename field.text to field.value
New methods EmailAddresses.add() and PhoneNumbers.add()
Minor fixes

- LastModificationTime -> LastModifiedTime
- Better initialisation of item properties in constructor
- etc.