dist/tools: add cppcheck script

enables easy linting of the whole tree or just one branches changes against
another
SQUASHME: file(line) -> file:line

Addresses https://github.com/RIOT-OS/RIOT/pull/1604#discussion_r16936195
SQUASHME: add documentation
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