Different Exception Type between Sync and Async client

[laucia]

Swagger-py's HTTPError is an exact copy of request's one
https://github.com/Yelp/swagger-py/blob/master/swaggerpy/exception.py#L9-L21
https://github.com/kennethreitz/requests/blob/master/requests/exceptions.py#L13-L31

but the SynchronousHttpClient throws requests.exceptions.HTTPError, and AsynchronousHttpClient throws swaggerpy.exception.HTTPError which makes error handling closely tied to the client used.

replacing the HTTPError code in swaggerpy/exception.py by a from requests.exceptions import HTTPError could solve this and by backward compatible.

[dnephin]

Duplicate of #100

[prat0318]

Closing it. Progress is tracked from now in #100.