fix license to LGPL

fixes #1155 needs @haukepetersen to review
unittests: exit(1) or error
added missing Licence in at86rf231 driver files.
Merge pull request #1162 from AlaeddineWeslati/master

added missing Licence in at86rf231 driver files.
Merge pull request #1159 from thomaseichinger/fix_license_sam3x8e

sam3x8e: fix license to LGPL
Merge pull request #1161 from Kijewski/unittests-exit-code

unittests: exit(1) or error
Merge pull request #1164 from Kijewski/unittests-use-subfolders

unittests: use subfolders for different tests
unittests: separate test suites from main.c

This change removes the need to patch the main.c if you add or remove a
test suite. A test suite in `tests/unittests/tests-XXX` needs to export
the function `void tests_XXX(void)`, which gets called by `main()`.

The `tests-XXX/Makefile` looks like your average module:
```
MODULE = tests-XXX
include $(RIOTBASE)/Makefile.base
```