Improve performance by using Paramiko+ssh rather than docker exec under the hood

[ChrisTimperley]

Pretty much every interaction in dockerblade involves issuing a docker exec call. On my system, the overhead of a single docker exec call via docker-py appears to be on the order of several 100ms. This is much too slow.

Instead, we could use docker exec to launch an SSH server inside the container. The dockerblade API could then be implemented by interacting with that SSH server via Paramiko rather than issuing docker exec calls.