Fire events when Provider execution is started and finished

Currently the XML reporter creates the report for a normal run. Then the same XML is written again by every rerun. This is not efective job.
Employing events specifying the begin and the end of the entire run would prevent from this happening.
If the rerun is not configured, the report can be created by test set completed as it is now.
If the rerun is configured, the reports should be produced at the end of the provider execution.