Fix error when raising without an Exception.

Raising without a valid exception is invalid:

>>> raise
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType
>>> raise Exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception

Changing this to "raise Exception" fixes this problem.
PyNUT: Create a custom exception class.

This maintains backwards compatibility, and allows calling programs to
use "except PyNUTError" instead of "except Exception" when using PyNUT
methods.

See https://wiki.python.org/moin/HandlingExceptions for more
information.
Change format of raise keyword.

Fixes PyNUT Python 3 compatibility.
Added author information, bumped version.

According to the semantic versioning scheme (http://semver.org/), adding
features that do not break backwards compatibility with previous
releases means that the minor version number should be incremented.
Fixed version description.
Merge pull request #129 from rshipp/python3

PyNUT: Python 3 support and better exception handling