Add category feature to the core.

Before this pull request, the 404 and 404-search templates where handled
as full HTML pages without the page context (layout).

With this pull request both pages are rendered inside the selected
themes layout. Additionally the 404 template now contains the "search
form" and the homepage link.

All Themes are already updated with the above changes.

This reverts commit 9fa8fab864.

Before this pull request, the 404 and 404-search templates where handled
as full HTML pages without the page context (layout).

With this pull request both pages are rendered inside the selected
themes layout. Additionally the 404 template now contains the "search
form" and the homepage link.

All Themes are already updated with the above changes.

Feature to change Sitemap priorities direct with the "config.ini" for
SEO. Additionally this commit adds the "lastmod" tag for posts.

- Set the Sitemap priorities for subtypes (Base, Post, Static, Tag, ...)
within the config.ini
- Values beween 1.0 and 0.0 or "false" to disable a subtype.
- Default priority is 0.5 as fallback. (see functions.php) The default
for "base" (Homepage) is 1.0.
- Disabled subtypes aren't available in the sitemap.xml (index).
- The sitemap for posts now contains the "lastmod" set the the post's
date.

Make the tag URL always lowercase and accept case insensitive URL e.g
/tag/htmly or /tag/HTMLy.

Cache the widget and refresh it when any content edited or new content
added.

Use htmlspecialchars() for title tag and description tag generation.

Adding popular posts widget.

Add new theme blog

Add bootstrap menu classes.

Trying to add content type: image, video, audio, link, and quote. No
styling yet.

Make HTMLy more flexibel.

Add post draft feature for the blog post.

Remove empty <p></p> tags.

Add options recent.count to the recent posts list functions so we can
change it using config.ini

Move the more link to template directly so we can modify the text
without changing any functions.

Move the title, description, and canonical from the head_contents for
better templating. Eg. for Twitter Card or OG.

Update the jquery to 2.1.4 and cleaning up the image upload codes.

Updating the copyright and the contributor list.

Styling, performance tweak, and bugs fixes.