Add google app engine support to Travis build setting.

Includes:
- enhancement to Travis build setting and tests to run google app engine related tests (or skip them if run with python 2.6).
- a fix to one test that was failing after the change to API.get return value.
- And a fix to way GoogleAppEngine deals with 4xx and 5xx http responses (urlfetch behave like requests but send_request implementation was raising a ValueError if the status code was something else than 200).