Restore non-hardcoded JWT

[shankari]

We now know the issues related to the bad JWTs earlier:

• the background sync was terminating too early, and
• we were not trying to refresh the token if it was initially NULL (6a8ac0c)

Now that these issues have been resolved, we can move back to the original plan with the non-hardcoded JWTs. Need to think about whether that is better (involves more native calls but gives us XHR functionality which can give us nice progress bars like
http://github.hubspot.com/pace/docs/welcome/

[shankari]

High level design:

• separate UI and silent auth
• for silent auth
  • have a function to read the auth token from the keychain (getToken)
  • have a function to refresh the token (refreshToken)
  • have a function to getValidAuth which reads the token and refreshes it if necessary
  • have a function to refreshToken which reads the token and forces a refresh if necessary
• In the remote calls, use getValidAuth
• In background sync, invoke refreshToken in parallel with the other calls so that we will have a refreshed token available for the next sync.

Actually, we may want to just do this for every remote push instead for every background sync. That will ensure that:

• we refresh even when the trip is ongoing
• when a trip is ending, we refresh in parallel with the geofence creation, so we can hopefully finish both tasks. Let us try it and figure out!!

[shankari]

we have separated UI and silent auth, and come up with a good interface in
e-mission/cordova-jwt-auth#17

We could change the javascript code to use getJWT instead...