Exclude example_project/example_module from packages

If nexus is a dependency in another project that also uses the common example_project pattern, nexus' example_project will be found first in the path when using 'python setup.py test', and thus installing nexus via setuptools (i.e. by having nexus in tests_require) breaks all tests for those projects, as they find nexus' example_project, not their own.

I don't *think* either example_* dirs should be in the sdist for nexus, as they are for nexus internal tests.