Merge branch 'gtest' into 'master'

Gtest

Add unit testing to wolf with googletest.

If the `BUILD_TESTS` option is enable in the top level `CMakeLists.txt` it will retrieve googletest from github and compiles it.

Then all wolf-tests present in the `test` folder are compiled in one single executable.
This executable can be break-down into smaller chunks - see `test/CMakeLists.txt`

Provide a very simple test as an example.

See merge request !82