vtimer: fix callback usage

There is no need to test the "handler" set in the vtimer struct, and
have some code executed then. We just can make the code to execute the
handler. To lengthy `if else if`, just a call.
Merge pull request #1034 from Kijewski/vtimer-callbacks

vtimer: fix callback usage
vtimer: fix vtimer_remove()
vtimer: minor style fixes
vtimer: unify and enhance interface

Currently the vtimer only accepts relative timestamps, even though it
uses absolute timestamps internally. This PR adds the parameter
`vtimer_absolute_t absolute` to every function, which is either
`VTIMER_RELATIVE` or `VTIMER_ABSOLUTE`.

`vtimer_sleep()` sleep uses `thread_sleep()` now instead of
`mutex_lock()` to allow spurious wake ups. A parameter `int *spurious`
is added, which is set to 0 if the wake up was not spurious.