Merge pull request #177 from gianninou/master

change C compilation (one file compiled), add C for recursion
Merge pull request #178 from gianninou/master

remove todo and compilation for bat
Move the repo containing the user sessions to github (and rework git sessions)

- That change is quite intrusive because I cleaned up the git data
  handling on my way, sorry for mixing up.
- I killed the LessonChanged event. Nobody cared and it made it
  difficult to save the exercise from which we are switching.
- It seems to work, but it needs more tests...
remove JAXP jar
remove C compilation for string