Improved SEO, including better support for meta tags

With these changes it is possible to define guides.json with custom description and keywords:

```
{
	"name": "dev_framed",
	"url": "dev/framed",
	"title": "Framed Editing",
	"meta_description" : "Some sample meta description",
	"meta_keywords" : "Some, sample, keywords"
},
```

If description is not set, it is created automatically based on the first text found in the document.