Fix two download suggestions in the docs:

1) On the quick start page provide a direct link to the downloads (suggested by @pbailis).
2) On the index page, don't suggest users always have to build Spark, since many won't.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #662 from pwendell/quick-start and squashes the following commits:

0622f27 [Patrick Wendell] Fix two download suggestions in the docs: