Model2Doc should provide a log API dedicated to the document generation process

We need, for a project, to write a log file in the workspace, for each document generation, that's why we want to create a log API dedicated to the document generation process in Model2doc.

We need to provide a logger API. This logger will be replace the calls to Activator.log.*** everywhere during the document creations (template2structure and structure2document processes). So we propose to add a 'logger' as new parameter for each method used by the process generation.

Model2Doc will provide a default implementation for the logger and will use in replacement of Activator.log.*** calls.

The logger API should be public to allow us to override it for our project.