[Fix #373] Add 'date' accessors

[joethorley]

We will also need some tests for this functions. Could you please add a few? Also please look at #355. I think using make_datetime and then as.Date might get us into trouble.

I have added some tests. Also I avoid the problem of using make_datetime and then as.Date raised in by pasting Year-Month-Day to a character and then using as.Date.

Note the date function provides the make_date function proposed in issue #355 (which @hadley suggested be called as_date)

For more discussion about the date and date<- functions see #373.

[vspinu]

Two notes on working with git:

• You don't need to open a new issue each time you re-write your commits. Just do git push --force. Github will update the pull request automatically.
• Each commits must deal with one issue. That makes it easy to navigate in the history. Could you please squash your two commits into one. If you are using R-studio or some other git-aware editor, there must be a way to do an interactive rebase and squash commits interactively. Otherwise from command line follow these instructions.

[joethorley]

Well I squashed but in the process I managed to introduce conflicts!
Sorry.
I'm going to cancel this pull request and retry fresh based on what I have learned!