Update CONTRIBUTING.md

Add comment about labels, hint at Development Procedures.
core/sched: clean up

- remove stray 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`
core/sched: fix pid/tcb & refactor SCHEDSTATISTICS

pid and tcb_t were compared instead of pid and pid

SCHEDSTATISTICS:
- reduce hwtimer_now calls
- dont use thread_last_pid anymore
- increase readability
core/sched: remove MODULE_NSS helpers (API CHANGE)

remove thread_getlastpid and last_pid from the API, it is unused
cpu/lpc2387: fix unused parameter warning

mark 'mode' as unused in pwm_init
Merge pull request #1405 from LudwigOrtmann/pwm_fixup

cpu/lpc2387: fix unused parameter warning
sys/vtimer: spin on short duration in vtimer_sleep

closes #571
native: remove stale TODOs and blank line

The values are quite OK.
Merge pull request #1376 from LudwigOrtmann/sched_fixup

core/sched fixup and optimize
native: enable reboot with tap

partially fixes #787
Merge pull request #1408 from LudwigOrtmann/native-cleaning

native: remove stale TODOs and blank line
Merge pull request #1410 from LudwigOrtmann/issue-787

native: enable reboot with tap
board/flash: changing flashrom_write signature

	→ changing all flashrom_write() function to the same signature
	→ adding const keyword to signature
dist/pyterm: enable unicode decoding

use unicode streamreader on serial interface
fixes #1371
pnet: Fix INADDR_ANY and INADDR_BROADCAST

INADDR_ANY and INADDR_BROADCAST should not be initializers.
Merge pull request #1414 from sgso/fix-inaddr_any

pnet: Fix INADDR_ANY and INADDR_BROADCAST
sys/shell: fix error when not using sixlowpan

```
/sys/shell/commands/sc_net_if.c:631:27: error: ‘IPV6_MAX_ADDR_STR_LEN’ undeclared (first use in this function)
             char addr_str[IPV6_MAX_ADDR_STR_LEN];
                           ^
```
make: model net_if dependency on net_help
make: replace spaces with tabs in Makefile.dep
vtimer: renamed e|dINT to disable|restoreIRQ