Porting to Drupal 8

Hey everybody,

today I started porting the XSL Formatter project to Drupal 8. I wrote some mails with dman and he added me as a co-maintainer.

Roadmap:

1. Move formatter code to own FieldFormatter class
2. Move XML XSL transformation to own utility class
3. Identify and perform other necessary drupal 8 changes
4. Describe a migration path

Follow up:

1. Create internal XSL entity
2. Move uploading XSL files to own configuration form which creates these XSL entities
3. Change FieldFormatter settings_form to provide only former uploaded XSL entities

Both list may not be final :-)