managed types can now be subclassed in python and override virtual methods
Merge branch 'master' of https://github.com/tonyroberts/pythonnet

Conflicts:
	pythonnet/src/runtime/runtime.cs
creating instances of classes that are managed types sub-classed in python from managed code now works
- Ensure python threads are always initialized, even if the main interpreter hasn't initialized them, otherwise attempting to dereference objects in finalizers called by the concurrent GC will fail.
- Dispose of PyObject instances after use in CreateSubType rather than wait for the GC
- Don't try and dereference derived class instances after python has shutdown if called from the GC
Merge pull request #1 from tonyroberts/master

pull subclass changes