Homepage preparation

We might want to start preparing the currently favored [proposal](http://ndr-qef.github.io/Caskroom/) for release. This PR is meant to kickstart the process of discussion and iteration, as kindly suggested by @rolandwalker.

Included here is the candidate as visible in the live github page. There are two more branches that you might want to merge later on: a test branch (which, as of now, only includes a baseline grid to be inserted as a background image to make sure that things line up properly), and a header-image branch for playing around with companion graphics.

A few starting points follow.

**Layout**. The page is somewhat awkward at mobile resolutions: the shell examples are moved before the section header, which can be confusing. We can either improve the separation between sections (e.g. by adding a border), or rework them so that the header is always on top.

**Content**. For the most part, I am fairly persuaded that the four sections cover Cask well, but we can improve the copy and shuffle things around should we feel so inclined. However, the "get Cask" section needs work.

**Documentation**. As it is now, it would make little sense to have a "docs" link in the bottom menu, since Cask docs are kept in markdown files divided by topic. It might be desirable to move most of their contents to a more easily organized wiki, which offers quick page creation and better sectioning. However, I assume that keeping them in the main repo was a deliberate decision; in that case, we could simply add separate links to each of them.

**Search**. It would be a good idea to offer a web search interface, as it allows prospective users to see what is available without installing Cask or browsing the repo manually. We might want to ask the mantainer of [Braumeister.org](http://braumeister.org) about the popularity of their service, and adjust our priorities accordingly.