Removing distinct from task query to see impact on Jenkins (initiated by http://forums.activiti.org/content/activiti-sql-query-performance)

Added documentation on setting process variables.

* Moved main documentation to VariableScope#setVariable() and
VariableScope#setVariables();
* Updated descriptions for RuntimeService#messageEventReceived(),
RuntimeService#setVariables() and RuntimeService#setVariable to point to
main documentation;

Added documentation on setting process variables for method messageEventReceived

messageEventReceived.

- Convience methods added to BpmnModelUtil

Changed logging IOException to the configured logger.