Plugin base classes: update method signatures & fix raise (#598)

This way IDEs can verify method overrides in subclasses, otherwise these
are flagged as erroneous.

Changed base classes to properly raise NotImplementedError; previously
they would cause "TypeError: exceptions must derive from BaseException"

Also fixed exception handling in sources.service.clean().

lemur/sources/service.py

@@ -198,7 +198,7 @@ def clean(source):
 
     try:
         certificates = s.clean(source.options)
-    except NotImplemented:
+    except NotImplementedError:
         current_app.logger.warning("Cannot clean source: {0}, source plugin does not implement 'clean()'".format(
             source.label
         ))