move dependencies to vendor

vendor/github.com/docker/libtrust/README.md

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+# libtrust
+
+Libtrust is library for managing authentication and authorization using public key cryptography.
+
+Authentication is handled using the identity attached to the public key.
+Libtrust provides multiple methods to prove possession of the private key associated with an identity.
+ - TLS x509 certificates
+ - Signature verification
+ - Key Challenge
+
+Authorization and access control is managed through a distributed trust graph.
+Trust servers are used as the authorities of the trust graph and allow caching portions of the graph for faster access.
+
+## Copyright and license
+
+Code and documentation copyright 2014 Docker, inc. Code released under the Apache 2.0 license.
+Docs released under Creative commons.
+