sys/net/ccn_lite/README.md change wording + add ACM ICN reference

sys/net/ccn_lite/README.md fix encoding
tests/periph_spi: let build fail without SPI
make: introduce and use periph_spi feature
core: refactor check for valid pid

move into header and remove redundant condition
core: add doxygen header for kernel_types
core/msg: add DEVELHELP checks for valid pid
Merge pull request #1794 from LudwigOrtmann/valid_pid

core: refactor check for valid pid
Merge pull request #1809 from LudwigOrtmann/Makefile.features-SPI

make: periph_spi feature
drivers: added low-level I2C driver interface
cpu/stm32f1: added low-level I2C driver
board/iot-lab_M3: added I2C device definitions
board/iot-lab_M3: added features
tests: added i2c driver test
Merge pull request #1785 from haukepetersen/add_stm32f1_i2c

cpu/stm32f1: added I2C interface&implementation
drivers: added driver for ISL29020 light sensor
board/iot-lab_M3: added ISL29020 interface defs
tests: added test for ISL29020 light sensor driver
Merge pull request #1786 from haukepetersen/add_driver_isl29020

drivers: added driver for ISL29020 light sensor
net: Initial import of a global packet buffer
Merge pull request #1716 from waehlisch/master

Update CCN-lite Readme