Add tox conf for coverage.

tox.ini

 [tox]
-envlist = flake8, pypy, py26, py27, py32, py33, py34
+envlist = flake8, pypy, py26, py27, py32, py33, py34, coverage
 
 [testenv]
 recreate=True
@@ -10,3 +10,10 @@ commands=python setup.py dev
 [testenv:flake8]
 deps=flake8
 commands=flake8 diceware tests setup.py
+
+[testenv:coverage]
+deps=coverage
+commands=
+  coverage erase
+  coverage run setup.py test
+  coverage report --include='diceware*' --fail-under=100