wx backend should not try to render jpeg or tiff if PIL is present

This patch has been tested under Windows, OSX, and Linux.  If PIL is present, it gives an accurate rendering of the graph, whereas the wx backend's print_jpeg and print_tiff don't.  I've attached before and after pictures (before and after this patch).

Other backends that might exhibit similar behavior, but I am not in a position to test, are:
* gdk
* gtk
* macosx

A test script that generates the images is below.
```python
#!/usr/bin/env python
import matplotlib
matplotlib.use('WxAgg')
import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0.0,2.0*np.pi,50)
y = np.sin(x)  

plt.plot(x,y,color='red')
plt.title("Any text can have math: $E = m\sqrt{c^4}$")
plt.xlim(0.0,2.0*np.pi)


canvas = plt.gcf().canvas
print type(canvas)
canvas.print_tiff('out.tif')
canvas.print_png('out.png')
canvas.print_jpeg('out.jpg')

plt.show()
```

Before,jpeg
![before](https://f.cloud.github.com/assets/1141744/178509/28de8d4a-7bbc-11e2-9dbb-d17429ec6d8b.jpg)
Before, png
![before](https://f.cloud.github.com/assets/1141744/178510/290ac7b6-7bbc-11e2-9ba9-2ab2f1723894.png)
Before, tif
[Could not display because file type unsupported...it looked very similar to the "before" jpeg]


After, png
![after](https://f.cloud.github.com/assets/1141744/178512/30c4d64a-7bbc-11e2-940a-f7b5d921236a.png)

After, jpeg
![after](https://f.cloud.github.com/assets/1141744/178513/30c37228-7bbc-11e2-9561-ecf77003431f.jpg)

After,tif
[Could not display because file type unsupported...it looked very similar to the "after" jpeg]