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()`.
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()`
drivers: Provide basic interface for all network devices
drivers: Make general IEEE 802.15.4 driver part of net_dev
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
net_dev: Provide some default values dependent on module selection
unittests: add test network device

The thinking behind this commit is twofold:
1. test the "lower" part (the device part) of net_dev
2. provide a well defined testing infrastructure for modules that utilise
   net_dev
The actual tests are provided in the following commit
unittests: Provide unittests for net_dev_dummy

* tests if net_dev_dummy is correct and if driver part of net_dev is
  correct
tests: Add functionality test for net_dev_base
sys: net: Initial import of a general interface to a network protocol