Small fix for RangeSelection:

While parts of the code refer to self.axis_index, other parts refer
to self._determine_axis(). According to the doc, self._determine_axis
is only ever used, when self._axis_index is not set. In fact, the
property getter for self.axis_index handles this case, so self.axis_index
should be used in all cases.

It will only affect the behaviour if indeed axis_index is beeing assigned
to something different than the usual value, which will hardly every be.
Nonetheless, I guess we prefer correctness.
Merge pull request #155 from burnpanck/fix-range-selection-manual-axis-index

Small fix for RangeSelection: