Improve/complete/refactor documentation.
Remove unused import
is_open -> is_open_to_user
Registration system
Refactor administration
Allow to force registration
Database updater
Merge pull request #22 from layus/feature/doc_improve

Improve/complete/refactor documentation.
Allow hooks to return values (and simplify the code)
Add template helper
Add hook for the admin menu (and a little refactor of the code for the menu)
Merge pull request #23 from GuillaumeDerval/registration_system

Registration system
autopep8 is aggressive
Contest system!
Default penalty for WA/TLE/RE should be 20 minutes
Merge pull request #25 from GuillaumeDerval/contests

Contest system