fixed Wformat warnings
sys: rename ringbuffer functions

Closes #1011.
Merge pull request #1092 from BytesGalore/fix_print_format_warnings_test_thread_msg

test:test_thread_msg fixed Wformat warnings
core: documentation: updated, improved, and completed doxygen comments
core: documentation: fixed bitarithm documentation
core: documentation: replaced task w/ thread
core: documentation: fixed typo in license header
documentation: extend description for CONST attribute
bitarithm: remove unused macros
documentation: fixed doxygen for schedstats
Merge pull request #1099 from Kijewski/issue-1011

sys: rename ringbuffer functions
arm_common: no needless `_gettimeofday()` warning

The `arm_common` syscalls implement `_gettimeofday()` for the benefit of
newlib. The syscall call only work if the `rtc` module or the `vtimer`
modules is transcluded. If neither module is used, a warning is printed
by means of `#warning`.

This warning is useless if the user does not invoke `gettimeofday()`.
Further this warning prevents the use of `-Werror`.

This PR puts the function in its own file, that will only be linked if
it was used. And if the function was used, then a link time error occur
if neither `rtc` nor `vtimer` was transcluded.
Merge pull request #1097 from Kijewski/gettimeofday-warning

arm_common: no needless `_gettimeofday()` warning
Merge pull request #983 from OlegHahm/core_doxygen_update

core: documentation: updated, improved, and completed doxygen comments
drivers: fixes to the low-level GPIO driver

- fixed doxygen comments
- added GPIO_UNDEFINED value
- added return values to all functions
Merge pull request #971 from haukepetersen/periph_gpio_fix

drivers: fixes to the low-level GPIO driver
Introduce macro to define files that should be deleted on make clean
Move .elf file to it's own target
Make directory creation in object compilation more makey
Add dash package