Add html meta tags / SEO

Currently we have:
```html
<meta name="description" content="DENKMAL.ORG - Basels Eventkalender. WaslOift am Rheinknie und wie hört es sich an?" />
<meta name="keywords" content="denkmal, basel, bs, denkmal.org, eventkalender, events, party, parties, ausgang, kultur, wasloift" />
```
Maybe have a different meta tag per page, mentioning the date?

By using the date in the url (instead of the weekday) we will probably lose our nice Sitelinks.....


![screen shot 2014-04-13 at 03 35 55](https://cloud.githubusercontent.com/assets/360800/2688552/171f9b8e-c2ac-11e3-857c-81a9b05d0c55.png)