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

setup.cfg

@@ -3,11 +3,7 @@ zip_ok = false
 
 [nosetests]
 exclude=medusa
-where=supervisor
 nocapture=1
-cover-package=supervisor
-with-coverage=1
-cover-erase=1
 
 [bdist_rpm]
 requires = 

tox.ini

@@ -5,7 +5,7 @@
 
 [tox]
 envlist =
-    py26,py27
+    py26,py27,cover
 
 [testenv]
 commands =
@@ -19,7 +19,7 @@ usedevelop = true
 basepython =
     python2.6
 commands =
-    python setup.py nosetests --with-xunit --with-xcoverage
+    python setup.py nosetests --with-xunit --with-xcoverage --cover-package=supervisor --cover-erase
 deps =
     mock >= 0.5.0
     meld3 >= 0.6.5