added support for path-level parameters

[dtao]

According to the spec Path Item Objects can include parameters:

http://swagger.io/specification/#pathItemObject

"A list of parameters that are applicable for all the operations described under this path. These parameters can be overridden at the operation level, but cannot be removed there."

This commit adds support for this scenario and modifies the existing tests to provide coverage for it.

[cyprieng]

Thanks for the contribution ! Can you just fix the test for python 2.6 ?

[flavianh]

Bump @dtao

[dtao]

Yes, I'll fix for Python 2.6.

[cyprieng]

Thanks !