tox.ini: Add "cover" target

```
(py26.venv)marca@marca-mac:~/dev/git-repos/sh$ tox
...
cover runtests: commands[2] | coverage report
Name    Stmts   Miss  Cover
---------------------------
sh        916    142    84%
cover runtests: commands[3] | coverage xml
cover runtests: commands[4] | coverage html
_________________________________________________________________________________________________________________________________________________________ summary __________________________________________________________________________________________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  cover: commands succeeded
  congratulations :)
```