When an exception has been handled when using the request / app context in a with statement, `sys.exc_info()` will still contain the exception information even though it has been handled already. The `__exit__` methods pass in `None` for the exception value in that case, which needs to be distinguisable from the default value for the `exc` parameter. Use a dedicated singleton sentinel value instead.
removed init_jinja_globals hack from app.py after consulting mitsuhiko
(didn't work on py 3.3 "as is")
removed with_statement future imports, not needed any more
needs more work on 2.7 as well as on 3.3