Add an AUTHORS file, simplify COPYING, bump year to 2017

Added:
- AUTHORS file listing Shaarli contributors
- mailmap information to group a Git author's different aliases
- Makefile target to list contributors from Git commit data

Changed:
- Simplify COPYING by using a single "Shaarli Community" entry
- Bump year to 2017

See:
- man git-shortlog
- https://www.kernel.org/pub/software/scm/git/docs/git-shortlog.html#_mapping_authors

Signed-off-by: VirtualTam <virtualtam@flibidi.net>