BF: skip some tests if no nipy/nibabel available

Detected on
http://nipy.bic.berkeley.edu/builders/pymvpa-py2.x-wheezy-sparc/builds/76/steps/shell_5/logs/stdio
http://nipy.bic.berkeley.edu/builders/pymvpa-py2.7-osx-10.8/builds/99/steps/shell_5/logs/stdio
BF(PY26): do not use dict comprehensions yet

Detected on:
http://nipy.bic.berkeley.edu/builders/pymvpa-py2.6-osx-10.5-ppc/builds/109/steps/shell_5/logs/stdio