ENH: Use python warnings module for template warnings

I changed the output of the warnings to go through the warnings module.
See : http://docs.python.org/library/warnings.html

The warnings are still sent to stderr by the warnings module.
The warnings are also formatted to not display the filename and the linenumbers, as this will always
display the same lines in itkTemplate.py

Users can now disable warnings with the following lines before importing ITK :
import warnings
warnings.filterwarnings("ignore")

or by running python with : python -W ignore foo.py
Merge topic 'Python-warnings'

16e346b7 ENH: Use python warnings module for template warnings