Allow to retrieve a token

Summary:
Passphrase can also store tokens, available in the material envelop
under the token property.

Test Plan:
A token is for example used by the cachet container to store the Laravel application key.

Ref T1433

./bin/getcredentials 126 token

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1433

Differential Revision: http://devcentral.nasqueron.org/D1761