Hide some components on ftw.book's reader view.

I've added a `book.scss` file which hides some components such as the footer on the reader view for gaining height. This heavily improves the reading experience since currently the height of the reader content area is quite small on smaller screens.

## Background 

The reader view detects the available view in the content area so that there is no page-wide scrollbar. This is important because within the reader content area there will be a scrollbar and having multiple nested scrolling areas is pain.

I see the reader view more as a kind of application which does not necessary to behave exactly the same as the rest of the website. Therefore it is ok to hide certain components for improving the user experience.

## Improvements

This pull request disables theese components:
- `ftw.footer`: The footer consumes a lot of height and is not necessary when reading a book.
- banner image: The `#banner-image` is usually not available on the reader view, but the div takes some space anyway.
- `div#viewlet-below-content`: the below-content viewlet is usually empty but takes some space anyway.
- `div.row.belowContent`: the belowContent area (theme) contains the "back-to-top" link, which is not useful since the reader makes sure that there is never a page-wide scrollbar. The link also does not work and makes no sense within the reader content area.

## Before
 
![bildschirmfoto 2014-03-10 um 16 08 49](https://f.cloud.github.com/assets/7469/2374995/f0598d84-a866-11e3-88ed-70a107287ba0.png)

## After

![bildschirmfoto 2014-03-10 um 16 08 56](https://f.cloud.github.com/assets/7469/2374998/f667914e-a866-11e3-82e8-e72b8e09e4e1.png)

## Further Improvements

I thought about hide some more components but I'm not sure about it:

- Breadcrumbs: I'm tempted to hide them too, but I think it is important for navigating back since the whole left navigation is reader-view-only and stays within the reader-view.
- Links outside of content area: Maybe we could also remove the links at the top and at the bottom, outside of the main area (grey background) so that we can increase the height of the whole content area. But I'm not sure if this is a good idea.
- Header: Maybe we could even make the header more compact?

@ninfaj could you take a look and tell me what you think about it?