Initial draft of HTTP streaming documentation

Problem

It's too difficult to learn HTTP streaming by reading the source, and
the test cases in EndToEndTests.scala are too alien looking to model
real world problems on.

Solution

Documentation and examples of usage.

Result

More people know about HTTP streaming and will be comfortable to try it
out.
I think it's about using the new 6.0 style to create a new instance, it requires to get rid of `ServerBuilder` and `ClientBuilder`. 
Example for client  : `def newClient(codec: Codec): HttpClient` or using `def withCodec(codec: Codec): HttpClient` . 
[Full details here](https://github.com/twitter/finagle/blob/master/finagle-http/src/main/scala/com/twitter/finagle/Http.scala#L97-L172)