Add authentication layer for model weights

[rossbar]

Implements the deepcell-users authentication layer for the model weights.

The _auth.py module is vendored directly from the deepcell/utils module for consistency across deepcell libraries. At some point in the future this should probably be extracted to its own package but that's a separate PR.

This still needs documentation+testing (along with everything else) but this at least gets the auth layer in place and should be working now. Quickstart to trying this out:

1. Create an account at https://users.deepcell.org (if you don't already have one)
2. Create an API key (if you don't already have one) and export it as an environment variable; e.g. (export DEEPCELL_ACCESS_TOKEN=<your-key>)
3. Run the example as in the README.