Moved gather_keys_cli.py and fixed import

This is how I fixed the issue #27.

I realized that the issue occurs only before the package has been installed, but I still think that it is better to avoid having possible import issues.
Even after the installation of the package, the older version of the script worked only because it imported `FitbitOauthClient` from the directory containing both `gather_keys_cli.py` and `api.py`. The dependencies of `api.py`, which were causing the issue, were then retrieved from the installed package.

With this fix the behavior is uniform and I think it is better.

The only thing that does not completely satisfy me is having the `gather_keys_cli.py` script at the root of the project directory but I don't know what is the pythonic way to deal with such situations.