Quit using the misleading function 'Token::start_with?'.

Fix #43.
Now 'Ex.3.1_2' isn't a child of 'Ex.3.1'.
Merge pull request #46 from chiro/issue-43

Quit using the misleading function 'Token::start_with?'.
Add a new type of reports: 'closed'.

Students cannot post 'closed' reports but super users can browse them.
Add an example of closed reports.