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.
Just thinking about the way of writing:
why not
```multi_site``` and ```current_site``` ?