Removed unnecessary override annotations form Account and EventSpot services.
Added commons codec dependency used for HMAC computation.
Added new constructor for Constant Contact in order to receive the client Secrect.
Provided also getter and setter for clientSecret property.
Added validation methods for billing change notification webhook.
Added BillingChangeNotification model.
Added WebHookValidator class.
Added error messages for "no client secret" and "invalid webhook".
No modification. Just properly formatted the code.
Code comment and cleanup.
Removed log.
Updated javadoc.
Moved webhooks implementation to separate package.
Revert "Updated javadoc."

This reverts commit 2b53752173520e8ce80551af3b15c5b69a664f97.
Added proper comments.
Updated javadocs.
Merge pull request #35 from halusstefan/master

Webhook Notifications