Make preview visible even if product isn't set to visible yet

Change readme from plain text to markdown while we're at it.

Change the URL to use the new controller with the secret hash key.
The hash key allows us to give the admin a "secret URL" to view
the product, without making the product completely publicly visible.
Module screenshot
Add installation and screenshot to readme
Merge pull request #2 from kalenjordan/kalen_livePreview

Make preview visible even if product isn't set to visible yet