Update to backend_wx

I have just committed an update to backend_wx.

New features:
- Preliminary support for interactive pan and zoom. This
  does not yet support use of mouse wheel, but buttons
  seem to work.

Bug fixes:
- Interactive sessions no longer require a PyCrust
  console (fixed by JDH)
- Location for pixmaps now defined by Distutils (fixed
  by JDH)
- Circle centres now correctly located.
- Axes background colours now respected
- Coloured text labels now functional
- Misc changes of variable names to conform to matplotlib

Known bugs:
- Scripts (and interactive sessions) do not exit cleanly
- Vertical text renders incorrectly (e.g. ylabels)
- pcolor almost works (rounding error)
- xlabel location incorrect
- No clipping on text labels

There are probably many other bugs - please inform via this list.