Automatic getter in case the date is present

Adding automatic getters for the day, month and year components in case
the date is present + added two new getters for the month.
Removed the initForCoder hack

initForCoder was reassigning self, which isn't a so good idea.
Moreover, the frame was somehow wrong after it. Hence I attempted to
remove dangerous code, mutualise the setup, and make it work for my own
project, at least.
Merge pull request #13 from onekiloparsec/master

Improvements for initWithCoder: and MZDay getters