I had a problem with PHP5.5 that caused ajax responses to be empty. This
fixes it, as the problem was related to pretty inconsistent calling to
headers, XMLWriter::startDocument etc. etc.
lib/plugin.php now has a parent onAutoload function that finds most common
files that are used in plugins (actions, dataobjects, forms, libs etc.) if
they are put in the standardised directories ('actions', 'classes', 'forms',
'lib' and perhaps some others in the future).
