Ensure data dir is created as parent to licenses dir.

Prevents error on startup::

  File "/home/safe/src/samplesdb/samplesdb/licenses.py", line 94, in
  __init__
      os.mkdir(self._cache_dir)
      OSError: [Errno 2] No such file or directory:
      '/home/safe/workspace/samplesdb/data/licenses'
Merge pull request #2 from safehammad/create_data_dir

Ensure data dir is created as parent to licenses dir.