As of VIM 7.3 it is no longer possible to specify the encoding in the
modeline. This gives an error message whenever such a file is opened,
thus this commit removes the enc setting from the modeline.

This fixes an infinite loop in breadcrumbs() and makes the behaviors in
all places where breadcrumbs are used consistent so that non-numeric
values, values < 0 and 0 are treated the same way.

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.

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

The media options dialog has currently an open bug regarding the
behavior of the nolink option. Additionally will wrong syntax be
inserted when using Internet Explorer < 8. Since this can't be
fixed on time, this patch disables the media options for the release.

The redirect target URL is constructed with escaped ampersands since b6258081.
Thanks to Chris Tapp for pointing out.