python3 portability fixes for pandas and the ipython notebook

This changeset tries to address the RDKit issue #326.

  * pandas was added to the travis-ci build environment
  * the open file mode in rdkit.Chem.PandasTools.LoadSDF has been changed from 'rU' to 'rb' (fixes an issue occurring with python3 during tests execution)
  * some string/bytes portability issues in handling the images in PandaTools and IPythonConsole have been addressed to fix depictions support in the ipython notebook (at the moment only spingCanvas was tested, so the quality is rather poor)
  * python -m compileall . was used on $RDBASE to locate residual syntax errors in code that is not exercised by the tests suite. print statements and exceptions handling were fixed (runtime errors were not checked).