tests/unittests: merge lib-ringbuffer into core

Fixup for https://github.com/RIOT-OS/RIOT/pull/2017
doc: fix drivers/srf02 documentation
doc: fix drivers/srf08 documentation
Merge pull request #2160 from PeterKietzmann/fix_srf_ultrasonic_doxy_warnings

doc: doxygen fix srf02/srf08 ultrasonic warnings
Merge pull request #2157 from LudwigOrtmann/unittests-lib

tests/unittests: merge lib-ringbuffer into core
test/driver_nrf24l01p: changed default GPIOs

not all boards have GPIO_x > GPIO_7, so the new values are
safer to use.
Merge pull request #2161 from haukepetersen/fix_test_nrf24

test/driver_nrf24l01p: changed default GPIOs
vtimer: set custom msg_t.type with vtimer_set_msg

Currently, when using vtimer_set_msg the corresponding msg_t is filled
with the MSG_TIMER ("12345") type.
This approach makes it difficult to differentiate between incoming
messages via vtimer_set_msg.

In this PR I introduce another parameter for the vtimer_set_msg
function to specify a custom msg_t type.
Merge pull request #2162 from cgundogan/vtimer_set_msg_param_type

vtimer: set custom msg_t.type with vtimer_set_msg