Create TruncateHTML.php

Originally from CakePHP but modified a few times on it's travels around the Internet.
Downloaded from http://www.gsdesign.ro/blog/cut-html-string-without-breaking-the-tags/
Update index.php

Added the ability to truncate the blog post displayed.
If the blog's config.php has an entry for 'preview_chars' under 'Blog' then it will only display 'preview_chars' characters peer blog entry on the index page.
Update Filters.php

Moved truncateHTML() here and renamed to blog_fliter_truncate()
Deleted apps/blog/lib/TruncateHTML.php
Renamed truncateHTML() to blog_filter_truncate() for consisitency.
Update config.php

Added preview_chars option
Update index.php
Merge pull request #197 from gareththered/master

Truncated Blog Preview