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
at86rf231: use radio_packet_length_t
cc110x: use radio_packet_length_t
cc110x_ng: use radio_packet_length_t
cc2420: use radio_packet_length_t
Merge pull request #1375 from OlegHahm/radio_packet_length

Radio packet length
examples: use DEVELHELP per default
doc: add detailed scheduler documentation
Merge pull request #1530 from LudwigOrtmann/examples-develhelp

examples: use DEVELHELP per default
Merge pull request #1441 from LudwigOrtmann/scheduler_documentation

doc: add detailed scheduler documentation
cpu: removing tabs from asm files (#1439)

```find . -name "*.[sS]" -exec grep -l $'\t' {} \;``` and ```ack --asm -l "\t"```
revealed three more files with tabs.
Merge pull request #1532 from cgundogan/remove_tabs_asm

cpu: removing tabs from asm files (#1439)
.astylerc: Respect max code line length of 100
Merge pull request #1536 from krf/fix-astylerc

.astylerc: Respect max code line length of 100