examples/pyidentity.py: Add a simple example that implements an identity-like element in python and passes buffers th...

Original commit message from CVS:
* examples/pyidentity.py:
Add a simple example that implements an identity-like element in
python and passes buffers through. It lacks buffer-alloc & query
handling at the moment, because the required gstreamer funcs aren't
wrapped.
* examples/sinkelement.py:
Make sure to call gobject.threads_init() in the example.