Use data installer for footer_links update

The current install script breaks the default Magento installation because it forces the creation of the `cms_block` table before the installer can - halting the script with `Base table or view already exists`.

* Move `footer_links` modification to data installer.
* Removing (now unused) classes/sql installer
* General cleanup (syntax, license headers, etc)

**Note:** This method has no dependency on a custom Resource Setup class and also only updates `<ul>` to `<ul class="nav nav-pills">` rather than overwriting the whole block content.