changed parameters form `kernel_pid_t` to `int`
dist/tools: add cppcheck script

enables easy linting of the whole tree or just one branches changes against
another
Fix thread_yield by avoiding the (too) early re-enablement of IRQ,
that is: before the newly selected thread's context is totally restored
Prevent the compiler to add an unwanted prologue in thread_yield()
Fix the race condition when an interruption occured at the beginning
of thread_yield(), i.e.: just after SR(R2) was pushed but before
the rest of the suspended thread's context was pushed!
SQUASHME: file(line) -> file:line

Addresses https://github.com/RIOT-OS/RIOT/pull/1604#discussion_r16936195
core: priority_queue: simplify inheritance
SQUASHME: add documentation
cpu: atmega: doxygen: blacklist upstream headers
Merge pull request #1642 from N8Fear/avr-libc-fix-doxygen

cpu: atmega: doxygen: blacklist upstream headers
SQUASHME: undo accidental commit of timex.h
SQUASHME: tool -> tools path typo
Merge pull request #1604 from LudwigOrtmann/cppcheck-script

dist/tools: add cppcheck script
vtimer: no needless type casting

Compare https://github.com/RIOT-OS/RIOT/pull/1639#issuecomment-54713048
and following discussion.
Fixed comments
tests: hwtimer shorten wait period

A sub-two-second period is needed for 16 bit timers.
tests/hwtimer: add self as author
Merge pull request #1643 from LudwigOrtmann/tests-hwtimer-shorten

tests: hwtimer shorten wait period
Merge pull request #1644 from Kijewski/vtimer-less-casting

vtimer: no needless type casting
Merge pull request #1639 from authmillenon/simplify_queue_inheritance

core: priority_queue: simplify inheritance