Make converting underscores optional in CommandManager

Doug,

Here is a patch for issue #28

This should hopefully retain default behavior but allow for the option to not convert.

The only issue I see is in add_command which doesn't apply the same underscore conversion that _load_commands does. Applying the same logic to that function would change the behavior of existing apps so I left it alone. Although it does a feel a bit inconsistent to have to remember that the conversion happens automatically in one place but not another.

Thanks
Joe