Customize privacy options on installation

This PR addresses #5052 and #6566 (as well as #6579 which is included in the first one).

It contains an update script for 2.9 that will disable IP anonymization if it wasn't configured before. The reason is that we change from "disabled by default" to "enabled by default", so we don't want to enable it on previous Piwik installs.

It also adds a new form in the last installation step. This form is dynamic and constructed with events so that there is no explicit mention of other plugins (as suggested by @tsteur), that means that plugins can add their own field here (but I guess it shouldn't be abused).

The form shows the default settings. The user can then submit the form which will saves his settings in the database. If he refreshes Piwik to get to the homepage somehow, then he will still use the default settings so all is good ("do not track" and "IP anonymization" are enabled by default).

![screen](https://cloud.githubusercontent.com/assets/720328/4890352/3d6e9f18-639e-11e4-9d77-4d05bf8f08f7.png)

Please review as it affects upgrade and installation, and I guess we really don't want to break those parts ;)
good for spotting this! :+1: