Added SBDebugger's log callbacks to Python-land

- Tweaked a parameter name in SBDebugger.h so my typemap will catch it;
- Added a SBDebugger.Create(bool, callback, baton) to the swig interface;
- Added SBDebugger.SetLoggingCallback to the swig interface;
- Added a callback utility function for log callbacks;
- Guard against Py_None on both callback utility functions;

- Added a FIXME to the SBDebugger API test;
- Added a __del__() stub for SBDebugger.

We need to be able to get both the log callback and baton from an
SBDebugger if we want to protect against memory leaks (or make the user
responsible for holding another reference to the callback).
Additionally, it's impossible to revert from a callback-backed log
mechanism to a file-backed log mechanism.

llvm-svn: 162633