poor man's unit testing

  * clipping appears broken: arctest.py, legend_demo.py, and

    > so on. This used to work, right?

    > * dotted lines are solid: see any plot with grid(True),
    > eg, log_test.py. Ditto for dashed lines: eg, plot(t3,
    > cos(2*pi*t3), 'r--') appears solid.

Both of these problems are arising from the select / unselect thing in
the GC. Since these changes were in response to the double gc problem
which no longer exists, I'm simply going to 'pass' in select and
unselect for the next release (which fixes both problems) above.
backend wx now passes all the tests, except the known issues.

I also added some more of the dpi scaling functionality to wx
(linewidth and markersize via the new renderer method
points_to_pixels). The last remaining area is dash size which is more
difficult since wx uses a constant for dash drawing rather than an ink
on/off approach. Does wx also support on/off? If I recall correctly,
you experimented with this but had a hard time getting it working right.

In any case the high res (eg dpi=300) wx output looks great.

There also seems to be an issue with getting the text bounding box
right with fontangles and fontweights that are not standard. To
experiment with this, uncomment 'if 0' on or around line 416 is
axis.py and run text_handles.py or text_themes.py. This will draw the
bbox around you text instance.