Remote SSH Unlocking

I've created a PR to add a simple SSH server that provides remote unlocking for LUKS devices protected with a passphrase. Please see #320. I do not believe the implementation is really "ready" for integration, however I've opened the issue and the PR to discuss the following: 1. Critiques of the implementation 2. Consolidating code paths (much of the unlock code is reproduced/taken from `init/luks.go`) 3. What anything else, such as documentation, unit tests, etc. need to be added to qualify for adding the feature into the source tree Thanks in advance for the consideration, as I really appreciate this project.