Forget previous global members in Bundle.update()

- Python's reload() or imp.reload() keeps old/non-overridden global variables in the module dictionary
  => if a class or method is renamed, its old version was still available with its old name
- This commit ensures that only special members ("__some_name__") are kept while using Bundle.update()
- This fixes #24
Removed the ununsed "stop" parameter of stop()
Bundle.update(): reset module dict members on error

If the call to reload() fails (syntax error, file missing, ...), the previous content of the module must be restored