Remove nose/rednose from `setup_requires` and instead declare them in
`tests_require`. Also explicitly add `nose` and `rednose` to
dependencies list in tox.ini (to avoid breaking test runs).
`python setup.py nosetests` is the preferred way for running tests, and
this works, except that placing test deps (nose/rednose) in the
`setup_requires` also means that these dependencies are pulled in for
installs of mongoengine. These deps are not actually be required just
to run mongoengine, so setup.py should not force users to install these
dependencies.
This refactoring should not change any test run semantics.
