Added ability to specify return codes.

[jbzdak]

When getting returncode other than 0 fabric raises SystemExit error by default, which is very unformtuate, since it is one exception you really shouldn't catch in Python.

You can of course use fabric.api.settings(warn_only=True, but you have to know of its existence, which can be hard. And we force user to use api other than provy api. I added a small feature that allows user to specify return codes that are considered as succesfull execurion, and if process return other return code I raise sane CommandExecutionError).

Let me know if any further changes are neccessary.

[jbzdak]

This requires some more work, it breaks some existing functionality.

For now it overrrides any warn_only setting set by any other code.