move DataBag to a single file.
added Delete method to delete a key.
refactoring for tests.
Redirect does not need to take a request object.
and added some utilities on request object.
time.Time is serialized in the different format when it is UTC.
null logger added for supressing logs in debug time!
we don't output any application logs in case of static file.
clarify the condtion scope of GetTime.
added User interface for requet context.
http.Roundtrip factory for GAE.
OAuth2 framework support!