tests: add test_vtimer_msg_diff

This test sets three timers and renews them periodically, showing the
difference they accumulate.
tests: test_vtimer_msg_diff: use array for timers, change first to 1s
tests: test_vtimer_msg_diff: add some timers, change max values
tests: test_vtimer_msg_diff: Makefile s/PROJECT/APPLICATION/g
native: remove some warnings about undef functions

This PR implements `real_X` for `X in (fork, dup2, unlink, execve)`.
These function caused warnings while making the default example.
Merge pull request #1332 from Kijewski/native-fix-warnings-in-default

native: remove some warnings about undef functions
tests: add branch option to license checker
travis: add license checking rule
Merge pull request #1325 from LudwigOrtmann/license_checker_travis

travis: add license checking rule
Merge pull request #1278 from kaspar030/test_vtimer_msg_diff

tests: add test_vtimer_msg_diff
tests: test_bloom: fix PROJDEPS concurrency

Our buildtests build one project concurrently for multiple boards.
The current creation of the `PROJDEPS` for `test_bloom` might fail if

* board `A` notices that the dependency is missing and gets preempted,
* board `B` creates the dependency and gets preempted,
* board `A` starts the creation but gets preempted in the middle of the process,
* board `B` works with a half complete created file.

This PR creates the dependency in the individual `BINDIR`.
Merge pull request #1338 from Kijewski/test_bloom-concurrency

tests: test_bloom: fix PROJDEPS concurrency
doc: fix comment indentation
tests: remove obsolete license pattern
tests: add new riot license header pattern
doc: replace RIOT license header with default lgpl