RF: move pylab import in viz/__init__.py into function itself

one function already had it, and otherwise nose gets confused for some
reason by importing viz/ submodule on one of the build bots

http://nipy.bic.berkeley.edu/builders/pymvpa-py2.6-xp/builds/121/steps/shell_5/logs/stdio

in the long run those functions might need to go into a corresponding
submodules anyways