Fix bug report for flashing qt object which has been deleted

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/veusz/windows/tutorial.py", line 845, in slotFlashTimeout
    self.flash.setStyleSheet('')
RuntimeError: wrapped C/C++ object of type QLabel has been deleted
Merge branch 'master' of github.com:jeremysanders/veusz
Fix crash in polar plot with reversed radial ranges. Support reversed scales.

Traceback (most recent call last):
  File "veusz/windows/plotwindow.pyc", line 904, in checkPlotUpdate
  File "veusz/document/doc.pyc", line 424, in paintTo
  File "veusz/widgets/root.pyc", line 123, in draw
  File "veusz/widgets/page.pyc", line 366, in draw
  File "veusz/widgets/widget.pyc", line 309, in draw
  File "veusz/widgets/nonorthgraph.pyc", line 153, in draw
  File "veusz/widgets/polar.pyc", line 265, in drawAxes
  File "veusz/widgets/axisticks.pyc", line 313, in getTicks
  File "veusz/widgets/axisticks.pyc", line 277, in _tickSelector
  File "veusz/widgets/axisticks.pyc", line 240, in _getBestTickSelection
ValueError: cannot convert float NaN to integer