Major commit: new major version

Major commit: new major version

1. APP can now be used to insert new exercises and remember the parameters
of the solution
2. the completed solutions are now recorded as XML trees and sent to the
server
3. new concept: class (collection of users)
4. new concept: standings (based on scores of solutions by others)
5. computation of scores revised
6. XSLT to map XML files to PDF and their visualization in the Web interface
7. redesign of the Web interface with improved layout and new functionalities