Export org.perfclipse.scenario package from core
Add UID for exceptions
Add runScenario method to Scenario manager.

* More logging added to both managers
* Better exception handling in both mangers (SchemaManagers now throws
  XMLSchemaException instead of SAXException, so the SchemaMangerTest
  was also modified.)
* Added dependency on PerfCake
Add Run Scenario to popup menu in Package Explorer

* Also move common parts of code with LoadHandler to Utils class.