Provide a guide how to use the HTTP Tracking API

Nobody has claimed this yet.

When going to the [Integrate](http://developer.piwik.org/integration) page we see a couple of Tracking guides. The first two links "JS Tracking Client" and "Tracking API Clients" list clients that access the HTTP Tracking API. The third link is a guide how to use Content-Tracking via the HTTP Tracking API but there is actually no guide how to use the HTTP Tracking API which would be helpful if people want to write a client or if they want to track data in their application without using any of those clients.

I don't think we need a huge guide here but we need to explain some basics and the link to the [Tracking API Reference](http://developer.piwik.org/api-reference/tracking-api).

At the end of the API Reference there is for example one part "Debugging the Tracker" which could be part of the guide. How to construct an example request could be moved there as well and explained in steps. We also need to explain when authentication is needed and how they can test it etc.

Start with the Integrate page and the Tracking API Reference, especially the existing “Debugging the Tracker” section. Outline a standalone guide covering example request construction, authentication, testing, and links to the reference; done means the guide is linked from Integrate and explains the basic HTTP Tracking API workflow.

Written by the indexing model from the issue text.