Updated Docker Client to accept TLS

The goal of this pull request is to enable Drone to connect to Docker using TLS. This would allow us to **securely** connect to remote Docker daemons and enable distributed builds across one or many servers.

- [x] ability to provide custom transport
- [x] ability to pass tls cert and key
- [x] remove hard-coded `http` from `Client.scheme`
- [x] cleanup the `New`, `NewHost` and `NewClient`
- [x] alter tls client and cert to be file content (byte arrays), not file paths
- [x] adequate testing