Initial priority implementation

- Fixed up some of the signal map for instrumentation
- Changed how priorites are specified -- no longer method name prefixes
  but dedicated decorators, including an easy way of specifying a custom
  priority level (no longer need to manually connect signals)
- Updated ExecutionTimeInstrument to work with the new system
- Also removed some dead code

wa/__init__.py

@@ -7,6 +7,7 @@ from wa.framework.exception import (ResultProcessorError, ResourceError,
 from wa.framework.exception import (WAError, NotFoundError, ValidationError,
                                     WorkloadError)
 from wa.framework.exception import WorkerThreadError, PluginLoaderError
-from wa.framework.instrumentation import Instrument
+from wa.framework.instrumentation import (Instrument, very_slow, slow, normal, fast,
+                                          very_fast)
 from wa.framework.plugin import Plugin, Parameter
 from wa.framework.workload import Workload