Update CONTRIBUTING.md

Add comment about labels, hint at Development Procedures.
core/sched: fixup and optimize

- Fix (non-)use of thread_*_pid - some errors were introduced by #1306, some existed before.
- Refactor SCHEDSTATISTICS
- Remove double spaces
- Remove TODOs:
    - MODULE_HWTIMER is not a module anymore
    - checking for NULL is necessary, at least without API changes.
      `sched_task_exit` sets `sched_active_thread` to `NULL`, then exits.
      Afterwards `cpu_switch_context_exit` calls `sched_run`.
- Eradicate MODULE_NSS specific code (*last_pid*)