Deprecation warning for drawing figures.

While running expr_mnist_SAE.py in the demo/ folder, this deprecation warning is reported:

/opt/lisa/os/lib/python2.7/site-packages/matplotlib/backend_bases.py:2407: MatplotlibDeprecationWarning: Using default event loop until function specific to this GUI is implemented
  warnings.warn(str, mplDeprecation)

Though it doesn't cause problem at the moment, but it is better to solve it.
