More readme content.
Various UI tweaks.
Merge branch 'master' of github.com:kentonv/sandstorm

Conflicts:
	website/apps/index.html
Add missing image file.
Web site tweaks.
Show a big green arrow pointing at 'My Files' after successful app installation.
More web site tweaks.
Add some info about debugging to the readme.  Also disallow passing bad package IDs to /install as a precaution.