drivers: Initial import of SPI low-level driver IF
drivers: fixes to the SPI low-level interface
driver: updated SPI driver interface
[SQUASH ME] added fixed speed values
[SQUASH ME] removed UNDEFINED from device list
[SQUASH ME] fixed signature of spi_transfer_reg()
[SQUASH ME] added begin_transaction, fixed doc
core: only store the stack size for DEVELHELP

`tcp_t::stack_size` is only examined by the shell command `ps` and
`DEBUG_PRINT`. For the latter one only if `DEVELHELP` was enabled.

This PR guards the member `tcp_t::stack_size` in `#ifdef DEVELHELP`.
Only if DEVELHELP was activated its value get printed by `ps`.

Closes #1287.
core: thread_measure_stack_free() is only useful for DEVELHELP
sys/transceiver: make transceiver_pid volatile

- because it is
SQUASHME: clean out extern kernel_pid_t transceiver_pid redefinitions
.astylerc: Respect max code line length of 100
Merge pull request #1536 from krf/fix-astylerc

.astylerc: Respect max code line length of 100
cpu/nrf51822: squash added support for nrf51822 cpu
boards/pca10000: squash added support for Nordic Dev Kit p1
boards/pca10005: squash added Support for Nordic Dev Kit p2
Merge pull request #1498 from fukuehl/nrf51822

cpu/board: added support for nrf51822
Merge pull request #1311 from haukepetersen/add_driver_spi_if

drivers: Initial import of SPI low-level driver IF
boards: fixup for pyterm renaming
Merge pull request #1543 from LudwigOrtmann/pyterm-fixup

boards: fixup for pyterm renaming