Various HTML & design changes

Hi Sebastian,

I saw your [call for contributions](https://plus.google.com/+SebastianBergmann/posts/DHuxptYmqTY) to the phpunit website and while I'm by no means a designer, some small tweaks to liven things up were easy enough to apply. Hope you like the changes ;-)

Smile,
Juliette


**Changelog:**
* Fixed a few HTML validation errors (unclosed html tags, type attribute for scripts and css and some other small tweaks)
* Removed loading of the prettify library files on the pages where it wasn't being used
* Removed request for non-existent `bootstrap-responsive.min.css` file: this file is no longer needed with bootstrap 3+ \[[Ref](http://bootply.com/bootstrap-3-migration-guide)\]

* Prettified the welcome banner
![welcome banner](http://snag.gy/Gx68f.jpg)
* Prettified the download blocks
![download blocks](http://snag.gy/W24gF.jpg)
* Prettified the documentation blocks
![documentation blocks](http://snag.gy/Ptu7T.jpg)