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.
native: implement hwtimer removal
ccnl: fixed commenting style
ccnl: eliminated empty translation unit warning
drivers - gpio: expanded gpio driver interface

- added functions for en|disabling a pins interrupt
driver - gpio: small adjustments
Merge pull request #1368 from haukepetersen/fix_gpio_disableinterrupt

drivers - gpio: expanded gpio driver interface
sys/ps: s/float/int for runtime

The high precision isn't needed, use per mille instead.
Circumvents printf problems on some platforms.

Also: prevent division by zero.
sys/ps: fix comment style, superfluous declaration
make: allow chosing ld-like over gcc-like options

make automatically adds '-Wl,' to linker-options. Supply `LINKFLAGPREFIX :=` to avoid that.
Merge pull request #1359 from backenklee/ldovergcclink2

make: allow chosing ld-like over gcc-like options
Merge pull request #1370 from OlegHahm/fix_stack_sizes

applications: set stack size correctly
Merge pull request #1352 from LudwigOrtmann/nofloat

sys/ps: don't use float for runtime calculations
Merge pull request #1351 from LudwigOrtmann/native_hwtimer

native: implement hwtimer removal
native: override INCLUDES w/ NATIVEINCLUDES

Currrently native overrides the object file targets, because it needs
the different include paths to interact with libc and the OS.

This PR simplifies their makefiles to only override the variable
INCLUDES, instead of overriding the targets.
setup git committer info in Travis

pkg/ Makefiles based on `Makefile.git` fail in Travis when attempting to apply patches with `git am` because no committer info is set.
This patch adds a dummy Travis username and mail in the 'install' step to satisfy git.
Merge pull request #1379 from benpicco/setup_git

setup git committer info in Travis
Extend CC2420 driver to comply with interfaces described in 'radio_driver.h'
Correction of HAL for CC2420 on concerned boards
core/msg: fix, optimize and improve

fixes:
fix race conditions by reordering dINTs
prevent null pointer dereference by adding forgotten target check
add forgotten eINTs
replace printf with DEBUG
fix debug messages

optimizations:
optimize pid access
reorder msg_send switches

improvements:
add debug statements
add missing return value to msg_send_to_self documentation