This patch implements what Adrian's patch "Hide logout button if auth
backend cannot logout" intended to do.

The 'logoff' capability was used to decide if a special method called
$auth->logOff() should be called when the user logs out, not if the
backend supports logouts at all. This was a superflous capability since
an empty logOff() method is implemented in the base class anyway - it
doesn't hurt to always call the method.

The 'logoff' capability is now deprecated. Backends who want to do
actions on logout simply need to overwrite logOff().

A new capability 'logout' was added which defaults to true. Backends
that can't logoff the user (eg. because they use some automatic
login/logoff mechanism) can set this to false.

Probably makes sense to add a 'login' capability as well...

Original author Sebastian Delmont agreed to rerelease the code
under BSD license to avoid license compatibility problems.

Other authors (me and Hakan Sandell) agreed as well.

This was an edit through the github interface which changed more
than intented.

This reverts commit 1720a8e9.

Behavioural changes:
  * buttons
    * no 'do=show' but clean URLs (show and back buttons)
    * the alias 'subscription' for subscribe buttons is now available
  * links
    * draft link has class 'draft', not 'edit'
    * revisions link has class 'revs', not 'revisions'
  * both
    * 'revision' is added as an alias for 'history'

These changes were mainly introduced to reduce the differences between
links and buttons.

Since an unchecked HTML checkbox has no value at all, a hidden field may be
used to specify an off value for the checkbox.

Since the specs aren't 100% clear, dates might be passed in different
formats by various XMLRPC clients. This patch makes date parsing a bit
more flexible.

Unit tests included.

according to the specs a value without a type is
recognised as string. This patch handles empty value
tags the right way as empty String.

This patch fixes a bug introduced in »code cleanup« #0b17fdc6