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.
added conv function from uint64 to timex_t and one minor bug fix
Merge pull request #1299 from phiros/timex_bugfix_and_improvement

Timex bugfix and improvement
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
make: refactor make system (parallelism, deduplication)

Almost everything was build sequentially in RIOT, because we employed
explicit for-loops to build directories (DIRS). This PR makes our make
system use normal dependencies to build directories.

All our compiling rules were duplicated, once for the application, once
for modules. This PR makes the application a normal module, removing
this duplication.
make: make DIRS usable for applications
make: create dependencies as side effect
make: use abspath for better error messages
make: evaluate the Git SHA only once
make: ensure order of `$(AR)` calls