make wx backends compatible with wxPython-Phoenix

This is based on the comments made to my previous PR for this.

Note that wxPython Phoenix snapshots can now be installed with pip:
pip install -U --pre -f http://wxpython.org/Phoenix/snapshot-builds/ wxPython_Phoenix 
On 12/5/2014 17:05, Benjamin Root wrote:
>
> In lib/matplotlib/backends/backend_wx.py:
>
> >          """
> >          Performs update of the displayed image on the GUI canvas, using the
> > -        supplied device context.  If drawDC is None, a ClientDC will be used to
> > -        redraw the image.
> > +        supplied wx.PaintDC device context.
> > +
> > +        The 'WXAgg' backend sets origin accordingly.
>
> technically, isn't it the wx backend? wxagg is a subclass.
>
The 'wx' backend uses the default value for param 'origin' to trigger 
the work around on MSW further down, and the 'WXAgg' sets it so the 
workaround is not executed.

Werner