Added a super basic "remember me" function.

Successful login remembers the username until the user chooses "log out" from the menu.

At least on iOS, the passphrase field focuses on start up and resume to make getting into the app a bit quicker when the username is remembered.

- System now has an `app.settings` stored in localStorage.
- Only current setting is `username`
Merge pull request #38 from devgeeks/issue-32-remember-me

issue 32 - Added a super basic "remember me" function.
Fixes #32.