modification to working with dates section

[Teebusch]

Minor changes to the "Working with dates section".

closes #730 closes #678

Details:

• Remove "As recommended earlier in this lesson, dates should be stored with one component per column". This wasn't recommended earlier in the lesson and I don't think this way to store dates should be recommended. I believe this closes #730. The other issue brought up in 730 (recommend CSV editor) has been settled by discussion.
• mention other lubridate functions . I think this closes #678. I kept it more general than suggested by @ramay If we want to introduce another specific lubridate function, we could do so as part of a new challenge (see #606)

[fmichonneau]

perfect, thanks!

The "as recommended earlier..." part comes from the spreadsheet lesson that starts the workshop but I think your fix addresses this well.