Fix lib/tasks/simulator.rake

Fixed lib/tasks/simulator.rake. The name generation was being done in such a way that you'd end up with conflicts. So I changed the naming scheme. And Mark.mark needs to be in a range of 0..4, so `m.mark = rand(criterion.weight)` would cause errors since criterion weights can vary.

```
Generating mark ...
Finish creating Student # 14337
Start generating Student # 14338...
A14/A14.java
Generating mark ...
Finish creating Student # 14338
Start generating Student # 14339...
A14/A14.java
Finish creating Student # 14339
Loaded suite /Users/danielstjules/.rvm/gems/ruby-1.8.7-p374@global/bin/rake
Started

Finished in 0.000234 seconds.

0 tests, 0 assertions, 0 failures, 0 errors
Daniels-MacBook-Air:Markus danielstjules$
```