Added a "Warning" class, and changed it so redeclaring or re-importing is a warning rather than an error.

    Because Pyrex has no #ifndef macro, it is impossibly painful to use
    pxi files for declarations in a large project.  SAGE is a large project.
    Also, in Python it is not an error to import a module twice.  Thus
    more in line with Python's behavior, multiple declarations of the same
    symbol is no longer an error.