use mutex in vtimer_sleep

this change prevents a race condition in case of
short sleep time

Fixes: https://github.com/RIOT-OS/RIOT/issues/550
Merge pull request #551 from mehlis/vtimer-mutex

use mutex in vtimer_sleep
use one consistent mail address in all places
Merge pull request #573 from mehlis/mail-address

use one consistent mail address in all places