BUG: Prevent leaks when factory methods are used from python

There is currently no automatic/clean mechanism to release the object
created by factory methods.

The only "hack" that exists for now is to decrease the reference
count manually in your code:

For more details, see [1], [2], [3] and [4]

[1] http://wiki.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/MemoryManagement
[2] http://massmail.spl.harvard.edu/public-archives/slicer-devel/2011/007513.html
[3] http://www.vtk.org/Bug/view.php?id=12139
[4] http://www.vtk.org/pipermail/vtk-developers/2011-April/009757.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22570 3bd1e089-480b-0410-8dfb-8563597acbee