Additional changes:
    - Implemented floor div (//) for the Scalar type
    - Trying to use a true div (/) on integer arguments will raise an
      exception
    - True div on unsigned integers is now coherent with the behavior on
      signed integers
    - True div on integers now results in floatX instead of systematically being
      float64
    - Added a few sanity checks in TrueDiv.grad
    - An exception is raised if one tries to use a constant of type 'long',
      because it is not clear what is going to happen
    - Fixed mean() of unsigned integer arrays