Extended user sessions

Closes #2;
a user may now log in to an extended session (does not expire via a session-only cookie);
non-extended sessions are now set via NDB_USERS_SESSION_STANDARD_DAYS global;
reformatting of indents to be more consistent;
added logging to 404 and 500 error handlers.
Update README.md

See #2; extended sessions completed!
Merge pull request #15 from roberthodgen/fix-2

Added extended user sessions