See http://forum.dokuwiki.org/post/22120

This changes fundamentally when metadata is rendered. This commit
introduces a new cache file for every page that just contains a
timestamp and is updated whenever the metadata of that page is rendered.

Metadata is rendered when p_get_metadata is called and the last
rendering has been before a page, metadata, configuration or renderer
update or purge is set like in the xhtml renderer cache.

Metadata is no longer automatically rendered when the xhtml renderer
cache isn't used but will still be rendered when needed as
p_get_metadata is called in the cache.

Metadata is also no longer rendered in the indexer script when missing
as that is already done by pageinfo() before anything else is done so
the indexer script won't be called when there is no metadata file.

Suggested by:   MQ <antinvidia gmail com>

Signed-off-by: Xin LI <delphij@FreeBSD.org>

Fixed a call to replace_invalid_with_pct_encoding (fixed upstream after SimplePie1.2) to continue to display correctly feeds with =

This patch only adds the capability to the Difference Engine lib, not to
the user interface, yet.

This patch does two things: It makes sure the native json_decode() will
honor the JSON_LOOSETYPE option of the class and it also adds way
to skip the use of the native function completely. The latter is
necessary for slightly non-standard JSON data.

p_purge_metadata now updates the metadata cache and the INFO array like the other metadata writing functions

this is mainly for use with plugin:upgrade and thus will be
cherrypicked into the current stable branch as well.

make custom buttons possible with html_btn() without the need of global $lang (more consistent with tpl_pagelink())