prevent rounding 0 if HWTIMER_SPEED > 1000000L

The current macros in hwtimer.h expect HWTIMER_SPEED to be < 1000000L, otherwise integer arithmetic will round the result down to 0.
Add a case to prevent that.
cpu/stm32f4: optimized GPIO driver implementation
SQUASH: cpu: fixed some typos
SQUASH: cpu/f4: changed order of irq enabling seq
cpu/nrf51822: changed ISR stack size to 512 byte
boards: adjusted TIMER_0 defines for NRF based b.

adjusted for
- pca10000
- pca10005
- yunjia-nrf51822
cpu/nrf51822: fixed timers

- changed TIMER_0 to be a 24-bit timer -> see Nordic PAN #32
- cleaned up timer initialization code
- added check if an interrupt channel was active
cpu: adjusted IRQ stack sizes for Cortex CPUs
cpu/stm32f0: fixed low-level timer driver
core: sched.c did not compile with DEBUG=1
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.
net_if: fix buffer overruns