Remove old-style assignment of raised exception

I've removed the old-style assignment of the ``Exception`` because
it is no longer supported in Python 3.x. Since it is not used, I've
removed it completely.
Switch to explicit relative imports as required in Python 3.x

The import of ``fields`` is an implicit relative import that is
no longer supported in Python 3.x. It has to be replaced with
``.fields`` to identify a relative import in the local package.

I've also added ``from __future__ import absolute_import`` to
force the same explicit behaviour in Python 2.x. This is
explained in more detail here:

https://wiki.python.org/moin/PortingPythonToPy3k#relative-imports
Merge pull request #2 from elbaschid/fix/support_for_python3

Add support for Python 3