[Website] Clarify FAQ and Downloads page regarding Hadoop dependency

As a first approach to reflect the results of the discussion here (https://github.com/stratosphere/stratosphere/issues/513) , I reworked the FAQ and Downloads page.

Preview Downloads page: http://robertmetzger.de/stratosphere/downloads/
Preview FAQ: http://robertmetzger.de/stratosphere/docs/0.4/general/faq.html

I'm happy to hear your feedback!