[commands] Allow loading cogs from folders.

Internally, instead of using module objects just use the `__module__`
attribute which is the same thing. From preliminary testing this seems
to work fine with both regular one-file-per-cog approaches and the
folder cog approach.

Fixes #126.

discord/ext/commands/core.py

@@ -155,7 +155,7 @@ class Command:
         signature = inspect.signature(callback)
         self.params = signature.parameters.copy()
         self.checks = kwargs.get('checks', [])
-        self.module = inspect.getmodule(callback)
+        self.module = callback.__module__
         self.ignore_extra = kwargs.get('ignore_extra', True)
         self.instance = None
         self.parent = None