docker-cloud: reorganize project to follow golang convention

- move pkg sources at the root of the repo
- remove `docker-proxy.go` to `docker-cloud-go`
- remove third party dependencies
- remove the build script
- update README to use markdown and simpler installation

People should now be able to install without manually cloning the repo or running some custom build script, just w/:
`go get github.com/brendandburns/docker-cloud`