add task variable methods

submit(taskId, variables);

+

/** creates or overwrites a variable value on the referenced execution */
Execution setVariable(String executionId, String name, Object value);

/** creates or overwrites the variable values on the referenced execution */
Execution setVariables(String executionId, Map<String, Object> variables);

/** retrieves a variable */
Object getVariable(String executionId, String variableName);

/** all the variables visible in the given execution scope */
Set<String> getVariableNames(String executionId);

/** retrieves a map of variables */
Map<String, Object> getVariables(String executionId, Set<String> variableNames);