Modified the common header statement for all tutorials, adding a link to download sandbox.

PREVIOUS SENTENCE
===================
Download to run this...

MODIFIED VERSION OF THE SAME
============================
Download the Hortonworks Sandbox to run this...

NEED FOR CHANGE
===============
The previous sentence does not say what to download.
Forgot to add what to download.  Adding "the Hortonworks Sandbox".
Merge pull request #19 from vivganes/master

Modified the common starting paragraph of all tutorials to make more sense.