Features/better placeholders

Adds a new page type called 'CUSTOM'. Custom pages behave almost like redirect pages except that they can have content and meta tags like normal pages. 

What can you use this for: To redirect to a custom controller that does whatever you want and still have CMS content along side it. Example usage in a host app:

In Custom Controller that page redirects to:
```ruby
  def step_one
    @page = Kuhsaft::Page.find(session[:kuhsaft_referrer]) if session[:kuhsaft_referrer]
    @rates_calculation_intro = RatesCalculationIntro.new(params[:rates_calculation_intro])
  end
```

View:
```
= render file: 'kuhsaft/pages/show'
```

This page then contains multiple forms that interact with the controler and are integrated via placeholders. 

**TL;DR: Custom Pages are a way of pimping placeholders with custom controller**