Tested account login and account creation

Note: had to remove the primary key property on UserID/QuizID (MySQL
throws an error if an unlimited TEXT column is set as a primary key).
Account login and account creation now works well. HomepageServlet
redirects to login page if no User-property is found in the session. I
also added a small test servlet for displaying messages to the user.

TODO today: List available quizzes, create quiz summary, and let users
take/create quizzes.
Added description property to quizzes in DB
Fixed SQLException when retrieving recently taken quizzes from DB

Also implemented some more test code to display popular quizzes,
recently created quizzes and recently taken quizzes on homepage
Implemented getURL methods for quizzes and users

Links directly to quiz summary or profile pages
Quiz summary almost fully implemented

Only missing quiz statistics and quiz options
Implemented Quiz options and average score/elapsed time

All functionality on the Quiz Summary page is now complete.
Added test questions, fixed some escaping, worked on TakeQuizServlet
Multi-page quizzes now work well

TODO: Implement immediate feedback option, and quiz results page
Implemented QuizResults, small fixes to DBConnection
Implemented "Immediate Correction" mode for multi-page quizzes
Added Activity.toHTML() and unified some code
Added queries to retrieve activities filtered by type

TODO: possibly unify some of the repeated code
Implemented friend activity list
Started work on Messaging and Friends
Implemented Friend Requests and Challenges

Messaging is about 70% done
Finished user profiles, messages and friends

The things we still need to implement are:
- Search for quizzes/users
- Achievements
- Create quiz
- Announcements (needs table in DB?)
- Admin page (can create announcements, remove users/quizzes, ban
userrs?)
- Frontend