BF: add mvpa2.viz to distribution.  Closes #143

Thanks robbisg for the report
ENH: adding shebangs and executable permissions to some examples missing them
BF: guard import of IPython.ipapi with externals.exists (for doctests safe harvesting)
BF: use our warning as the last resort anyone trying to find removed ChannelDataset
DOC: added title parameter
NF: scatter plot between underlay and overlay intensity
BF: make sure that identity transformation works (to show examples of poor alignment)
RF+BK: chance "-o" parameter from overwrite to output directory, to conform to other command line functionality
ENH: raise an error if any command line commands fail. Addresses https://github.com/PyMVPA/PyMVPA/issues/139
BF: do not crash if pprocess is not there and nproc=None
ENH+PY3: use // for integer division
PL: indent
BF: do not import hdf5 when external not present
PY3+RF: Improved Python3 compatibility
ENH: print error message with "%s", not "%r"
ENH+NF: improved and new __str__ functions
ENH+BF: proper conversion of linear indices to integer array
ENH: check for NaNs in coordinate equality comparison
ENH: more use of "%s" rather than "%r" in error messages
BF: type (forgot "x" in "xor")