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
Allow Mac linebreaks in shell

QEMU uses Mac style newlines in its UART implementation.
This change allows Unix ("\n"), DOS ("\r\n") and Mac ("\r") style
linebreaks.