ENH: Make TubeTK Examples testing dependable on TubeTK_TEST_EXAMPLES

     Added an advanced option TubeTK_TEST_EXAMPLES that is OFF by
     default. If ON, then the Python virtual environment will be
     set up and the IPython notebooks in the Examples folder can
     be tested.

     Also adjusted the README.md file and switched back and forth
     between installing matplotlib with pip or easy_install.