cpu/stm32f0: fixed low-level timer driver
pktbuf: optimize size overhead in packet buffer
core: put more intelligence into `queue_msg()`

Fixes #1942.

There were two instances were it was not checked the target thread has a
message queue before queuing the message.

This PR centralizes the check into `queue_msg()`.
Merge pull request #1961 from Kijewski/issue-1942

core: put more intelligence into `queue_msg()`
Merge pull request #1959 from authmillenon/pktbuf-opt

pktbuf: optimize size overhead in packet buffer
unittests: Add missing semicolons to macro calls
make: remove downloader "axel"

Breaks for `make -B`. If the file already exists, then `axel` fails
because it does not find the status file for the download.
Merge pull request #1929 from Kijewski/no-axel

make: remove downloader "axel"
Merge pull request #1972 from authmillenon/unittests-semicolons

unittests: Add missing semicolons to macro calls
unittests: make test macros save for strict aliasing
Merge pull request #1973 from authmillenon/test-macros

embunit: make test macros save for strict aliasing
c++: sys: add extern C to header files
Merge pull request #1793 from BytesGalore/extern_C_in_headers_sys

sys: add extern "C" to headers
nativenet: add missing include
core: always set sender_pid in msg_send_int
Merge pull request #1977 from authmillenon/nativenet-include

nativenet: add missing include
Merge pull request #1956 from haukepetersen/fix_stm32f0_timer

cpu/stm32f0: fixed low-level timer driver
6lowpan: free fragment buffer in error case
6lowpan: fix buffer overrun in decompression
IPv6: allow complete in ipv6_addr_init_prefix

If the specified prefix is a full IPv6 address, no bits remain.
Fixes a possible buffer overrun.