Add expected .csv columns to the db import help

[lgpage]

It would be useful to have an indication of what .csv columns are expected in the respective db import commands help information (nbgrader db assignment import -h and nbgrader db student import -h).

[jhamrick]

For reference, the expected columns correspond to the column names of the database tables: http://nbgrader.readthedocs.io/en/stable/api/models.html

These correspond to:

• assignments
  • name
  • duedate
• students
  • id
  • last_name
  • first_name
  • email

[jhamrick]

@mpacer this is something you could probably do now too, now that you figured out how to get the column names!