Made error checking much more user configurable.

The choice of which gphoto2 return values should raise an exception is
very program dependent, I've added an 'error_severity' dict which the
user can use to assign a different severity to selected return values.
There is also an overall 'error_exception' value that sets how severe an
error has to be before an exception is raised.
Moved 'use_python_logging' function.

Moved it from SWIG .i file to Python __init__.py file. This has required
a slight change to the Python helper classes. The Context class
constructor parameter 'use_python_logging' has been renamed to
'with_python_logging'.
Corrected typo.
merge upstream branch

will test new logging asap