FIX #149: Stop importing my package on `setup.py`.

* build: Extract version with execfile() on a separate `_version.py` file.
See https://github.com/matplotlib/matplotlib/blob/master/setupext.py#L105 for an example of what I mean.  If `_version.py` grows extra lines (like someone bulk-adding copyright statements or an accidental blank line) the current code will break.