Package reorg, variable renaming, bugfix for chpw

- The package was split into two modules: cli and db, with
  some other reorgs. The database access constants are now
  at the module level (in db) and the Store class is now
  named File.

- Variables that used to be named pw_store are now pw_db.

- chpw needed to pass a lambda as the password function,
  not the result of calling getpass.getpass().