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
Merge pull request #1373 from Kijewski/easify-nativeincludes-usage

native: easify NATIVEINCLUDES usage
Merge pull request #1223 from rousselk/cc2420-extended-driver

CC2420: modify&extend driver to comply with API described in 'radio_driver.h'
Merge pull request #1285 from LudwigOrtmann/msg_fixup

core/msg: prevent null pointer dereference
core/sched: fix (non-)use of thread_*_pid

some errors were introduced by #1306, some existed before
core/sched: refactor SCHEDSTATISTICS
core/sched.c: MODULE_HWTIMER is not a module anymore
core/sched.c: remove double spaces
FIXUP: ifdef last_pid for NSS and document it

also fix logic around it