The goal of this ticket is to modify the tracker API so that the cid parameter should no longer require to use the token_auth.

Tasks:

• modify tracker API
• update the reference docs http://developer.piwik.org/api-reference/tracking-api as well as any FAQ or user guide that may mention cid.

This overall goal is to make Mobile Apps Tracking easier to use and less need of configuration in the SDK. See also: #4994, Android SDK: #4733 and iOS SDK