Improving password encryption seeding algorithm

* Modifies userauth_local helper to use $user->created_at for password
  seeding.  This is far more unpredictable than $user->id, especially
  for the root administrative user, in the event of a data compromise
  or rainbow table attack.
* Falls back to old seed method if $user->created_at fails.  In this
  case, the user will be upgraded to the improved seed method.
* Closes #88.
Fixing but in PDO MySQL database driver

* Fixes bug in MySQL driver for database_pdo helper where multiple
  databases with the same table names would cause $driver->getSchema()
  to malfunction.  Fixes #87.