Implemeting target assistants

- Workload's update_result stage has now been broken up into two parts:
  extract_results and update_output. This is to allow the assistant to
  pull output from the target in between the two stages.
- Updated assistant implementations for Linux and Android targets from
  the exisiting code.
- Extended target descriptor code to handle assistants and their
  parameters as well.
- Updated  the target manager to actually make use of the assistants.

wa/framework/workload.py

@@ -66,10 +66,16 @@ class Workload(TargetedPlugin):
         """
         pass
 
-    def update_result(self, context):
+    def extract_results(self, context):
         """
-        Update the result within the specified execution context with the
-        metrics form this workload iteration.
+        Extract results on the target
+        """
+        pass
+
+    def update_output(self, context):
+        """
+        Update the output within the specified execution context with the
+        metrics and artifacts form this workload iteration.
 
         """
         pass