Replace inline CSS

 - Move generate_css function from viewlet into browserview
 - Updated cacheing of returned CSS file
 - Add timestamp to the generated CSS file url

Changes theme css integration from:
```HTML
<style type="text/css" media="all">... (like 45 000 chars) ;-) ...</style>
````
to:
```HTML
<link rel="stylesheet" type="text/css" href="/Plone/customstyles_css?ts=735370">
````

@buchi @jone 

closes #74