basePath is not mandatory now and op scope composing will work without tags

This pull request aimed to solve 2 issues:

(1) when basePath  is not provided (it is not mandatory according to swagger v2 spec) there is an exception
```python
/pyswagger/scanner/v2_0/patch_obj.py in _path_item(self, path, obj, app)
     38         """
     39         """
---> 40         url = app.root.host + app.root.basePath + jp_split(path)[-1]
     41         for c in PathItemContext.__swagger_child__:
     42             o = getattr(obj, c[0])

TypeError: coercing to Unicode: need string or buffer, NoneType found
```

(2) app.op is not populating if there are no tags in the path