This patch removes the need for requestFactories and decorators
by implementing http.RoundTripper transports instead.
It refactors some challenging-to-read code.
NewSession now takes an *http.Client that can already have a
custom Transport, it will add its own auth transport by wrapping
it.
The idea is that callers of http.Client should not bother
setting custom headers for every handler but instead it should
be transparent to the callers of a same context.
This patch is needed for future refactorings of registry,
namely refactoring of the v1 client code.
Signed-off-by: Tibor Vass <tibor@docker.com>
