SEC-2327: Document SecurityExpressionRoot
SEC-2404: CsrfAuthenticationStrategy creates new valid CsrfToken
SEC-2404: Fix CSRF config tests
SEC-1932 Added PBKDF2PasswordEncoder.

 - Also moved some logic into a new class, AbstractPasswordEncoder. Both PBKDF2PasswordEncoder and the now-simplified StandardPasswordEncoder extend AbstractPasswordEncoder.
 - Added tests for PBKDF2PasswordEncoder .