Adding a .gitignore file
Reorganising the repository + Lazy eta expansion

  -- Reorganisation
  It was starting to be a bit messy so here is a reorganization
  of the repository. Contribs are now clearly isolated in a
  dedicated directory, they have their own test suite.

  -- Lazy conversion test
  Adding a conversion test in Contribs/Model.hs which eta expands
  lazily + corresponding test cases.