Respect multi threading flag for Python

The current Python annotator can be compiled as single threaded or multi-threaded.

Many complex Python packages (e.g., NumPy) cannot run on a multi-threaded Python.

UIMA recognizes some annotators need to be single-threaded through a flag on its descriptor.

This flag should be honoured for the Python annotator, using a single-threaded or multi-threaded Python interpreter depending on the flag.

Depends on #8 .