Fix file save error on Windows

I ran the test suite on Windows 7 using the Anaconda environment and encountered an issue where I could not save `jpg` files using the `qt_plugin` in a `novice` test.  So, I replaced the `jpg`s with `png`s.  I put a debug point before `qimg.save` per [stackoverflow](http://stackoverflow.com/questions/7152597/qt-unable-to-save-jpeg-files).  Note that I did NOT have PIL installed (and the same happens on Linux Mint 17 when I uninstall PIL).

```python
(Pdb) im = QtGui.QImageWrite('test.jpg')
(Pdb) im.write(qimg)
False
(Pdb) im.errorString()
PyQt4.QtCore.QString(u'Unsupported image format')
```

I had a second error with `test_repr_png_roundtrip` in `io\test_images.py`.

```python
TypeError: 'QByteArray' does not have the buffer interface
```

So, I also made a change to `qt_plugin` as well.