NotSuccess error now has http.Response field

fix testutils go.mod

Get added, appengine.go removed

go1.11 module

Added ErrUseDefault error for use in Func and refactored.

Funcs that return nil, nil will now create an error.  Funcs
must now return ErrUseDefault to select default client.

added examples and updated README.  Removed log.go and moved
logic to testutils package.