Lots of stuff

Mostly feature-complete at this point, just need some more student
functionality.
Updated database schema

Added session_tag to sessions table, and start_time and end_time to the
onlineresponses table
Student Stuff

Students can now change their password and registration requires
usernames to be non-empty and unique.
fixed bug with checksessions.php, added (undocumented) feature to zipsessions.py to call this function, fixed month/week boundary when listing sessions