Add display name field to SIS User Import

The display name (short_name) field is optional. If absent, short_name
will behave as before. If present, it will explicitly set short_name to
the given value, unless it's a stuck SIS field of course.

Previously, if first and last names were not given, the user's name will
be a single space. The behavior is the same now, except when short_name
is present. In that case, an error will be thrown.
Remove error for setting short_name w/o first+last names
Update CSV documentation to include short_name
Rearrange add_user args to maintain compatibility
Merge pull request #438 from sfu/feature-sis-user-short_name

Add display name field to SIS User Import