Writer service

Create a Writer service that will be taking care of writing entities to a spreadsheet independently of the VBO plugin. The VBO plugin would then use the service to perform the export. Ideally we should probably have a Writer service, that would take care of converting the entities and writing as data into the Spreadsheet, and an Export service, that would take care of preparing the file and calling and configuring the Writer service to execute the conversion/writing. But for now, until the architecture of the module takes a stable shape, let's keep everything in the Writer service.