pthread: pthread_barrier should call yield
tests: clean up test_pthread_barrier Makefile
core: add `thread_get()`

Remove PID check duplication in `thread_getstatus()` and
`thread_getname()`.
board/stm32f3discover: fixed LED macros
cpu/stm32f3: let LD3 blink on hardFault
Merge pull request #1576 from haukepetersen/fix_stm32f0_leds

cpu/board: fixes to stm32f0(discovery)
drivers: improved the low-level GPIO driver IF

- removed UNDEFINED value from device enum
- changed most functions to void return value
- add parameter for interrupt callback
cpus: adjusted uart implementations for changed IF

- adjusted stm32f0
- adjusted stm32f3
- adjusted stm32f4
- adjusted sam3x8e
- adjusted nrf51822
tests/periph_gpio: added test cases for GPIO driver
Merge pull request #1484 from haukepetersen/fix_drivers_gpioif

drivers: improved IF for low-level GPIO driver
Merge pull request #1562 from Kijewski/the-first-two-commits-of-the-branch-issue-198

posix: minor pthread_barrier clean up
pyterm: fix exit behavior if twisted is not available
Merge pull request #1582 from N8Fear/fix_pyterm_twisted

pyterm: fix exit behavior if twisted is not available
Merge pull request #1580 from Kijewski/thread_get

core: add `thread_get()`
core: introduce KERNEL_PID_FIRST and KERNEL_PID_LAST
core: let valid PIDs start with 1