Add 'Crosswalk screens' article
Add details about how to install sample game
Add missing explanatory sections
Minor tweaks after proof reading
Add page about screen measurements
Update samples pages with space-dodge-game description

The space-dodge-game developed in the "screens" tutorial is
in the crosswalk-samples repo, so update the samples page
with details.
Merge pull request #135 from crosswalk-project/screens-1285

Add article about screen management