Handled tRNS color palette for PNG images (was crashing before)

Just fixing an issue with python 3.
Basically, you cannot do indentation mixing spaces and tabs in the same python file (https://www.python.org/dev/peps/pep-0008/#tabs-or-spaces)

Added bob.io.image.load function that is able to automatically detect the image type (using imghdr.what)

C++interface

Updated interface to adapt to available data types and color/gray images; added C++ interface documentation

Fixed issue #9