phpBBthreadfetch by PM 2Ring

PM 2Ring has been so kind to contribute a very useful phpBB forum thread scraper to our project. It is known to work on Linux and OS X. As far as I can tell the software only imports standard modules, so it should work on Windows, too.

As-is, the code is not installed by `setup.py` yet, and the `myentity.py` module is undocumented. I will shortly push two commits to fix that. After that I will give people one day to point out any issues that I may have missed. If everything seems right, I will then continue to merge phpBBthreadfetch together with my fixes.