Implement a crypto plugin to interface with HSMs via PKCS

Implement a crypto plugin for Barbican to interface to an HSM device. This plugin will use a PKCS #11 interface to communicate with the HSM, allowing for PKCS-compliant HSMs to be utilized with Barbican. This will be the secure plugin implementation for the M3 release.

See the original blueprint here: https://github.com/cloudkeep/barbican/wiki/Blueprint:-Plug-in-Encryption