Add JPEG as an image format for inline backend if PIL/pillow is available.
update what's new
Move testing for PIL[low] in the _figure_format_changed(...) function instead
of testing it at module startup. The test is only run if figure_format is jpg.

In help/error messages, refer to PIL/Pillow (i.e. not just PIL).
Forgot to import TraitError.
Saner acceptable range for jpeg quality parameter.
Better docstrings.
A somewhat better whatsnew
add unittest for jpg, only activated if PIL/Pillow is installed
fix docstring for pylab jpg unittest
use testing decorators to enable tests on module availability
add default value for select_figure_format's quality parameter
Merge pull request #4679 from dbarbeau/jpg-inline

JPG compression for inline pylab