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
Merge pull request #2650 from syngron/master

Lightsource shade method parameters for color range definition