Store passphrase in browsers local storage

@barrydegraaff I did some hacks to store passphrase in browsers local storage instead of storing in LDAP in plaintext (IMHO, it is an insecure way. Please see: https://github.com/barrydegraaff/pgp-zimlet/issues/43)
If you find this new commit somehow useful, please feel free to merge it into the main trunk.