added a specific validation for signature validation failures
update change log
Merge pull request #66 from Shopify/add_validation_exception

Add validation exception
Update README.md

Fixed step 4 to match the verification described in the official docs.
http://docs.shopify.com/api/tutorials/oauth
Fixed the phrasing
Added the missing comma
Merge pull request #68 from nCrazed/patch-1

Update README.md