Multisite

This pull requests implements multisite abilities into Fork CMS

![screen shot 2014-07-17 at 16 55 12](https://cloud.githubusercontent.com/assets/1398405/3614722/66db29fc-0dc2-11e4-9bb1-99210f9fd0ad.png)

This means a lot of changes to Fork

* New Multisite module added. This contains the CRUD to manage sites (and their languages)
* Multisite and Currentsite object added to the DIC. They are used to fetch information about the current site or about other sites
* SITE_DOMAIN is now fetched from the url and not from the parameters.yml file
* Pages are now linked to a site_id and a language.
* Content blocks are now linked to a site_id and a language.
* Search indices are now linked to a site_id and a language.
* Faq questions and categories are linked to a site_id and a language.
* Formbuilder forms are site dependent
* Location module is now site dependent
* Mailmotor groups are now added per site and language
* Permissions are extended with permissions per site/language combination
* Locale module saves all translations by default to the main site.
* Locale on a specific site falls back to translations for the main site.
* An analytics profile can be added per site
* Module settings can be site dependent.
* Next to the language switch, a site switcher has been added (if applicable)
* prepare_for_reinstall now has the option to delete your database tables
* ...

A lot of refactorings where done too, to keep the code base clean, even with the extra functionality.

This seems to be a complete multisite implementation. It would be awesome if somebody could test this. If there is feedback about bugs or missing functionality, I'll add them to the pull request.