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
pyterm: enable TCP connection to remote host
cpu: adjusted IRQ stack sizes for Cortex CPUs
cpu/stm32f0: fixed low-level timer driver
pyterm: make mostly pep8 compliant
pktbuf: optimize size overhead in packet buffer
drivers/periph/rtt: added overflow callback
Merge pull request #1950 from OlegHahm/pyterm_tcp_remote

pyterm: enable TCP connection to remote host
cpu/nrf51822: adjusted to RTT interface changes
cpu/stm32f1: adjusted to RTT interface changes
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()`.
core: sched.c did not compile with DEBUG=1
Merge pull request #1960 from haukepetersen/enh_driver_rtt

drivers: added enhanced RTT driver interface
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"