Make nose2.__version__ available for use.

Most python modules (including nose2's predecessor, nose) have a `__version__` variable that can be introspected for the convenience of other developers and users.

I suggest adding `__version__`.  Here is one possible (and simple) way to do it.

Without an internally stored version variable, it becomes difficult to identify which version you are actually using, when multiple versions are in the `PYTHONPATH`.

I look forward to your thoughts.