Creates and keeps the log directory for storing logs.

The log directory must be present for MIDAS to run:

    MIDAS/midas[master]% python launcher.py
    Traceback (most recent call last):
      File "launcher.py", line 104, in <module>
        launch_modules()
      File "launcher.py", line 101, in launch_modules
        spawn_module(module, current_lang, mod_name)
      File "launcher.py", line 81, in spawn_module
        file_handler = open(log_file, "a")
    IOError: [Errno 2] No such file or directory: '/Users/josh/code/python/edge/MIDAS/midas/log/example.log'

This pull request creates a log directory and a `.gitkeep` file inside so the directory will be present on a fresh clone of the MIDAS repo.