also capture stderr in capture plugin

This change adds the option --capture-stderr which allows to also capture stderr with nose. In case of a failed test the captured stderr will be shown below the captured stdout.

This has also been requested here https://github.com/nose-devs/nose/issues/305

It might be possible to make the code more elegant but I'm not sure about the original intentions of the current code so I didn't do any refactoring.