Implement very basic sshkey retrieval mechanism.

This follows more or less the bare minimum convention expected by the "From an
HTTP Endpoint" section described at
https://coreos.com/docs/cluster-management/setup/cloudinit-cloud-config/#toc_17

This allows for specifying users with autoconfigured ssh public keys based on
keys already provided to coreos-ipxe-server.
Added api documentation for new ssh key retrieval mechanism.
Merge pull request #10 from mattikus/ssh_key_server

Implement ssh key retrieval API endpoint