Fix multiprocessing tests in python2.7

Closes #28 for python 2.

Apparently, process pools are not contexts in python 2.7.
On the other hand, in py2 you cannot pickle instance methods, so
you have to implement Pool tasks as independent functions.

tests/plugins/async_plugin/asyncplugin.py

@@ -3,13 +3,15 @@ from senpy.plugins import AnalysisPlugin
 import multiprocessing
 
 
-class AsyncPlugin(AnalysisPlugin):
-    def _train(self, process_number):
-        return process_number
+def _train(process_number):
+    return process_number
+
 
+class AsyncPlugin(AnalysisPlugin):
     def _do_async(self, num_processes):
-        with multiprocessing.Pool(processes=num_processes) as pool:
-            values = pool.map(self._train, range(num_processes))
+        pool = multiprocessing.Pool(processes=num_processes)
+        values = pool.map(_train, range(num_processes))
+
         return values
 
     def activate(self):