infer_shape does not survive through all exceptions anymore, and now uses ShapeError instead of NotImplementedError to catch situations where a shape cannot be computed

Using a custom exception for integer divisions (safer), and fixed crash due to this newly raised exception

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

Added the option to hide warnings about cuda not being found by setting config.cuda.root to an empty string

debug mode such to ignore memory buffer provided to the linker to store its
outputs in them.

mq is nice, but sometimes it screws up where the patch gets applied...

later harder ( we would need to check if two scan have the same
mode_instance class or the same instance when we decide to merge??)