Added vmin and vmax arguments to LigthSource shade method which, if set,
changes the lower and upper border of the colorspace. Compare with
method contourf where these parameters are available.
Corrected code style.
Code syntax correction.
* Exchanged LightSource shade parameters vmin and vmax with norm
  parameter.
* Added LightSource test case similar to example
  http://matplotlib.org/examples/pylab_examples/shading_example.html
Code style fixes.
Another style fix.
- Simplification of norm usage in shade() method

- basic example and assert statement in LightSource test
MacOSX backend supports 2x DPI images and MathTeX.
Make sure self._idle is set to `True` in all cases

Even if `draw` raises an exception.
add try block in backend_gtk.py

added block around `draw` in `idle_draw` which can raise an
exception and will leave the window in a bad state.
added try block to backend_gtk3.py

Added try block around `draw` in `idle_draw`.  If `draw`
raises an exception the figure is never marked idle again and
pyplot functions will not work as expect.
added try block in backend_tkagg.py

Added try block around `draw` in `idle_draw`.  If `draw`
raises an exception the figure is never marked idle again and
pyplot functions will not work as expect.
Merge pull request #2665 from piannucci/retina

MacOSX backend supports 2x DPI images and MathTeX.
Merge pull request #2650 from syngron/master

Lightsource shade method parameters for color range definition
Merge pull request #2677 from tacaswell/fix_2583

Make sure self._idle is set to `True` in all cases