The form API gets a little mangled by Braintree JS (see
commerce_braintree_js_submit_form_submit) so raw
$_POST data does get used in place of
$form_state['values'] to retrieve the encrypted values from the form submission.
The Braintree server-side API is then loaded via Commerce Braintree, and used to send the encrypted data to Braintree's servers.
Installation instructions are available on the project page:
git clone --branch 7.x-1.x email@example.com:sandbox/tmsimont/2092519.git