Add exponential backoff to the login with polling
Add exponential backoff to the login with polling

Try to find an optimum timeout adding an extra delay of 2^n * step.
Simulating a 3G connection with the current settings and the
local provider behind Shiboleth (which adds 2 redirection) the timeout
stabilizes around the 7000ms.

Issue edina/fieldtrip-cobweb-project#85

edina/fieldtrip-sync,edina/fieldtrip-sync