Added a set of utilities for automatic grading, and updated documentatio...

Added a few utilities intended for automatic grading, namely:

- a mechanism to establish per-assignment deadlines and extra days for late submissions
- a mechanism to establish a per-assignment limited number of submissions
- a way to create a report as a text file, which is written into the user folder (so is visible from the web interface). The report by default is written immediately, but a delay can be configured.
- a way to set the score of a student, and macros to perform tests and add penalties if these tests fail
- a scribbling documenting these features

I've tried to integrate as smoothly as possible with the design of the handin-server, but for convenience when writing the report I defined two new parameters in main.rkt, namely:

- assignment-name: bound to the folder name of the assignment corresponding to the submission processed by the checker

- user-assignment-directory: bound to the root of the user folder corresponding to the current assignment (e.g. "Assignment 1/tester")

Finally, there is some layout changes in some files, which occurred after I used Command-I shortcut in DrRacket, so I /guess/ it should be fine...