Issue with passing variables to and from a sub-workflow

[sgehrig]

Due to some issues within \ezcWorkflowNodeSubWorkflow it is not possible to pass variables from the parent workflow to a sub-workflow and vice versa.

There are three distinct problems:

1. When a sub-workflow is resumed variables from the parent workflow are not mapped to the variables names in the sub-workflow (based on the variable mapping configuration): zetacomponents/workflow/src/nodes/sub_workflow.php:166
2. When the waitingFor variables from the sub-workflow are being fed back to the parent workflow the names are again not mapped back to the variables names in the parent workflow (based on the variable mapping configuration): zetacomponents/workflow/src/nodes/sub_workflow.php:191-194
3. The \ezcWorkflowNodeSubWorkflow::passVariables() method does not check whether a variable exists in the from workflow resulting in an exception if a variable is mapped but not yet set on the from workflow.

I'll provide a pull request to fix these issues.