Fix tox "cover" target and run by default

Previously, tox "cover" target failed for me on nosetests 1.3.0 with:

    nose.plugins.xcover: ERROR: You can not use both --with-xcover and
    --with-coverage. Using --with-xcover implies --with-coverage

because there was a confusing mix of nose options in both setup.cfg and
tox.ini.

I moved the coverage-related stuff from setup.cfg to tox.ini and now the
tox "cover" target works for me with nosetests 1.3.0