Timeout

[erwindon]

Is your feature request related to a problem? Please describe.
When there is a session timeout, most API calls end with "401 Unauthorized".
SaltGUI carries on because it may not have access to a single API call (or range of API calls).

Describe the solution you'd like
When a 401 is received, the current time should be compared with the session-end time as received at login. When the current time is past that, it is safe to conclude that the whole session has expired. The logout procedure should be triggered so that cached information is removed and the screen reverts to the login screen.
Function isAuthenticated can be retired.

closes #251

[erwindon]

Note that this branch is a sub-branch of options (so it cannot be merged now).

[erwindon]

@lostsnow @dawidmalina @elipsion
Do you have any experience with nightmare?
(i.e. the functional regression-test that runs as part of runtests.sh)
The regression test now fails and I cannot find how to get the proper error message...

[lostsnow]

Try this: mochajs/mocha#2025?