Convert unicode emails to byte strings for CSV grade reports

Unicode characters in emails were causing grade report generation to fail.

@ormsbee 
It's coming from the Django model field, so it should always be unicode. In any event, if it were a non-ascii bytestring, the `unicode()` invocation here would fail anyway with a `UnicodeDecodeError`.

Please just leave it at `student.email.encode('utf-8')`. I guess we might as well do the same for the username as well. I think @jbau was at some point looking into having non-ASCII usernames...?