Prepare release, improvements, and bugs fix  #226

Add new config: static.frontpage = "true" to enable the static
frontpage.

Use the pagedown extra instead of standard pagedown. Adding
sitemap.category.xml

Add category feature to the core.

Updated robots.txt + new htaccess files.

I made the following changes for the robots.txt and .htaccess files.

# robots.txt updated
- Allow theme assets (CSS, JS, Images, Fonts) for SEO
- Allow folder with uploaded images (/content/images)

# .httaccess files added
- /content - Deny all
- /content/images - Allow only jpg|png|gif
- /cache - Deny all
- /system - Deny all
- /system/admin/editor - Allow all
- /system/resources - Allow all
- /themes - Deny all
- /themes/blog/css - Allow all
- /themes/blog/fonts - Allow all
- /themes/blog/images - Allow all
- /themes/blog/js - Allow all
- /themes/clean/css - Allow all
- /themes/default/css  - Allow all
- /themes\default\img - Allow all
- /themes\logs\css - Allow all

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.

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

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

Adding popular posts widget.

Fix the date and time on the edit content page so it will displayed as
year, month, day, hour, minute, and second.

Add bootstrap menu classes.

strip_tags and preg_replace problematic tags.

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.

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.

Adding image upload to the markdown editor using
jQuery.AjaxFileUpload.js.

Styling, performance tweak, and bugs fixes.