Replace gevent with tornado

Closes #28

Added:

* Async test (still missing one that includes the IOLoop)
* Async plugin under tests. To manually try async functionalities:
```
senpy -f tests/
```

tests/plugins/async_plugin/asyncplugin.py

@@ -0,0 +1,21 @@
+from senpy.plugins import AnalysisPlugin
+
+import multiprocessing
+
+
+class AsyncPlugin(AnalysisPlugin):
+    def _train(self, process_number):
+        return process_number
+
+    def _do_async(self, num_processes):
+        with multiprocessing.Pool(processes=num_processes) as pool:
+            values = pool.map(self._train, range(num_processes))
+        return values
+
+    def activate(self):
+        self.value = self._do_async(4)
+
+    def analyse_entry(self, entry, params):
+        values = self._do_async(2)
+        entry.async_values = values
+        yield entry

tests/plugins/async_plugin/asyncplugin.senpy

@@ -0,0 +1,8 @@
+---
+name: Async
+module: asyncplugin
+description: I am async
+author: "@balkian"
+version: '0.1'
+async: true
+extra_params: {}

tests/test_extensions.py

@@ -167,7 +167,7 @@ class ExtensionsTest(TestCase):
         assert len(senpy.plugins) > 1
 
     def test_convert_emotions(self):
-        self.senpy.activate_all()
+        self.senpy.activate_all(sync=True)
         plugin = Plugin({
             'id': 'imaginary',
             'onyx:usesEmotionModel': 'emoml:fsre-dimensions'
@@ -205,3 +205,14 @@ class ExtensionsTest(TestCase):
                                     [plugin, ],
                                     params)
         assert len(r3.entries[0].emotions) == 1
+
+    # def test_async_plugin(self):
+    #     """ We should accept multiprocessing plugins with async=False"""
+    #     thread1 = self.senpy.activate_plugin("Async", sync=False)
+    #     thread1.join(timeout=1)
+    #     assert len(self.senpy.plugins['Async'].value) == 4
+
+    #     resp = self.senpy.analyse(input='nothing', algorithm='Async')
+
+    #     assert len(resp.entries[0].async_values) == 2
+    #     self.senpy.activate_plugin("Async", sync=True)