Added the Bulletin class and added it to Competition; Competition now
has UIDProvider in order to manage UIDs.
CompetitionEventHandler now has BULLETIN_ADDED and BULLETIN_REMOVED.
Useless ProblemSet commit (whitespace error); ProblemSubmission now
obtains its ID from Competition rather than a constant and
ProblemSubmissionInfo has been reflected to show this.
Merge branch 'bulletin' of https://github.com/blacksmithgu/scl into bulletin