Hopefully this patch will solve a few issues that have just been raised
about the new behavior of org-export-html-preamble/postamble.

`org-export-html-preamble' defaults back to `t', to reproduce the
previous default behavior.

`org-export-html-postamble' default to 'auto -- which mean that
org-export-email/author/creator-info will be honored by default.
When this is set to `t', the postamble will be inserted from the
formatting string in `org-export-html-postamble-format'.  When 
set to a string, this formatting string will be replaced by this
string.

Both variables also support functions, as org-export-html-preamble 
and org-export-html-postamble used to (but that wasn't documented).

Please test carefully and tell me if this is okay to apply.

Thanks!