Several fixes

* Refactored BaseModel for efficiency
* Added plugin metaclass to keep track of plugin types
* Moved plugins to examples dir (in a previous commit)
* Simplified validation in parse_params
* Added convenience methods to mock requests in tests
* Changed help schema to use `.valid_parameters` instead of `.parameters`,
which was used in results to show parameters provided by the user.
* Improved UI
    * Added basic parameters
    * Fixed bugs in parameter handling
    * Refactored and cleaned code

example-plugins/noop/noop_plugin.py

@@ -0,0 +1,5 @@
+from senpy.plugins import SentimentPlugin
+
+
+class DummyPlugin(SentimentPlugin):
+    import noop