Recommended fixes and signup flow improvements

### What's new

Some minor changes:
- changed module category to `advertising_marketing`
- updated module description
- sign up page tweaks
- use of fancybox for screenshots

The bulk of changes in this PR are to support a streamlined onboarding procedure for new stores. With these changes we can now:
- automatically fill in store information for their new LoyaltyLion account (e.g. store name, language, currency, PS version)
- insert their LoyaltyLion token & secret into their PrestaShop store with one click
- create hundreds of voucher codes in PrestaShop with one click

The latter two methods work by opening up a pop-up window to their LoyaltyLion module configuration page, passing it custom parameters like `ll_set_token_secret` which the module now picks up.

Instead of checking if ```LoyaltyLion_Connection``` has already been been defined by a previous require, you should use *require_one**. This test would not be needed anymore.

For more information: http://php.net/manual/en/function.require-once.php