Added nose to test setup.py.

Most people don't use 'python setup.py test' so 'tests_requires' is
included as part of install_requires at installation time. Furthermore,
running 'python setup.py test' will install testing packages locally in
the CWD; doing so will clutter the CWD and may introduce limitations as to how to upgrade testing packages.
Merge pull request #24 from yeukhon/setup-py-addition

Added nose to test setup.py.