Changes for consistence and compatibility in index.html and demo.html

Changes in the "index.html" and "demo.html" to provide more consistence
(same tab size, remove the final "/" in some tags, etc). Charset metatag
above the title for compatibility reasons (endorsed by HTML5 Boilerplate,
among others). Also, viewport metatag simplified to more concise
presentation, use & fast understanding.

So, the changes are:

- Code padronization (tab distance, "/" in some tags, etc)
- Charset tag above the title tag
- Simplification of the viewport tag