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: {}