Some search engine optimization for the website.

Updated title of start page from "Stratosphere > Overview" to "Stratosphere > Next-Generation Big Data Analytics".

Extracted static meta description and keywords from layout. 
Set individual descriptions and keywords for the most important pages.

Search engines (at least Google) penalize identical description and keyword meta data on multiple pages.

This should give us a better search result entries (title + description) and hopefully also better keyword coverage.

This PR resolves issue #411