Use sessionStorage to store the index container

This allows logging off (either via timeout or user action) but successive logins will show the index immediately from the sessionStorage cache.

This cache is cleared when the app is existed (either via quitting the desktop app or exiting the app in android via the back button or from the task switcher)
Use a SHA hash of the username in the key instead of the ACTUAL username
Explicit logout clears any sessionStorage
Whoops... needed the hex of the hash
Merge pull request #105 from devgeeks/sessionstorage-index

Use sessionStorage to store the index container