Reviewed and splitted the persistence guide

#22

- the **MVC** guides are no more
    - The "HTTP Request Handling" has moved to the "How Piwik Works" category
    - Controllers and Views have moved to the *Web Interface* category, along with pages and all
    - "Piwik APIs (Models)" has been moved to the *HTTP Reporting API* category and hase been renamed to "Piwik APIs" (it had nothing to do with a model layer)
- for the "Persistence & MySQL" guide:
    - #39 I have removed the SQL `CREATE TABLE` and merged the pertinent information (e.g. explanation of indexes or table naming strategies) into the "Entity" descriptions (where we list each property of each entity): it cleared up things a lot
    - I have extracted "How plugins can persist data" into a separate "Plugin Data" guide
    - I have renamed "Persistence & The MySQL Backend" to "Piwik Data"

Here is what the menu looks like now:

![capture d ecran 2014-12-17 a 16 55 18](https://cloud.githubusercontent.com/assets/720328/5466318/84c3b24a-860d-11e4-9208-607fe03cdcaf.png)

Hopefully now it's clear what these articles contain…