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.
ccnl: fixed commenting style
ccnl: eliminated empty translation unit warning
core/msg: revert part of #1285

use m->sender_pid instead of sched_active_pid again
undoes the optimization intended by #1285
fixes uint/int warning
core/msg: set m->sender_pid before it is used

fixes `msg_send_to_self` condition
SQUASHME: do it right
sys/transceiver: fix t9r_register API compliance
sys/transceiver: implement transceiver_unregister
sys/transceiver: refactor un/register

remove race conditions
make less restrictive (allow double-un/registers)
make loops more readable
sys/transceiver: use size_t instead of u8 in loops
Merge pull request #1395 from LudwigOrtmann/transceiver_unregister

sys/transceiver: add transceiver_unregister and refactor transceiver_register
make: move exported make vars to one place
make: don't use export on example makefiles
Merge pull request #1109 from Kijewski/make-exports

make: no "unfriedly" `exports` in the application's Makefile
Merge pull request #1387 from LudwigOrtmann/msg_fixup

core/msg: clean up after #1285
Merge pull request #1034 from Kijewski/vtimer-callbacks

vtimer: fix callback usage
make: rename VERSION into RIOT_VERSION

Building MSP boards gives an error, because `VERSION` is somewhere
defined in their toolchain as an integer.

This PR renames `VERSION` into `RIOT_VERSION`, because that's what it
is.
Merge pull request #1400 from Kijewski/riot-version

make: rename VERSION into RIOT_VERSION
Merge pull request #1362 from OlegHahm/ccnl_eliminate_warning

CCN_lite eliminate warning
Update CONTRIBUTING.md

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