Chaining FROM ./path in Dockerfile

[lk4d4math@gmail.com: add support for `FROM ./path`, `FROM git://url` and `FROM http://url/to/Dockerfile`]
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

[teabee89@gmail.com: remove git:// and dockerfile-url support for Dockerfile FROM keyword]
[teabee89@gmail.com: rebase + gofmt]
Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
can we add more info:

```
`<path>` must be the directory within current build's context which contains
Dockerfile. Note that `./` is necessary if you want to specify `<path>`. The
new `build` will be started with the context in `<path>`, and the resulting
image will be used when its done.
```