Changed the values recognized by the general config parser.
If value in config is a boolean it is recognized as boolean, not if it
is a string.

Fixes #718

New Python Django application default to PBKDF2 with SHA256 as a
password mechanism. This adds support for that mechanism in our
password hasher class. This will be needed in the tests for the new
PDO auth plugin.

autor duplicated + < and > fixes.

Permit three values:
- always (default), the other input field is always displayed
- exists, the other input field is only displayed when the setting
          contains value(s) not listed in choices*.
- never, the other input field is never displayed. If the setting
         contains any value(s) not listed in choices they will be
         discarded on saving.
* This means, under normal circumstances the admin will not see
  the other input field.  It will only appear after manual editing
  to enter a non-listed _choice or if an update were to remove a
  previously selected _choice