* NpgsqlContentProvider.csproj: new circle provider
* CircleManager.cs: initializes the default provider
* CircleProvider.cs: Makes abstract the CircleProvider class
* CircleInfo.cs:
* CircleInfoCollection.cs: cleans imports
* CircleManager.cs: implements a circle manager
* CircleProvider.cs: Defines a default Circle provider
* EventPub.cs:
* CalendarController.cs: refactoring
* CircleController.cs: implements with default circle provider
* FrontOfficeController.cs: throws descriptive exception message at
searching for a catalog
* Catalog.aspx: Fixes links on product categories
* ProductCategory.aspx: Fixes links on products
* YavscModel.csproj: Includes Circle provider definitions
