Plat 556 http rest proxy watching

When using this to watch (eg, etcd) it's difficult for debugging to have to catch exceptions, and we need to get data as it comes, not only once the connection is closed.

These changes allow for the HTTP Rest Proxy to be used in this context.
I don't particularly like this interface, because it basically says : "get the body from the response, unless you provide a callback, in which case the body will not be in the response, even though we still provide one as return value". Also, in general callbacks are use with event-based apis, that is when you do not know when they are going to be called. And this is useful only for this specific use case.
How I would do it, to the extend that curl allows it, it to provide the header via the Response with a flag that says that more body parts are to come. Then a "fetch next part" method which would fetch the rest until the response is done. That would respect the blocking paradigm of the class.
Now, I really doubt puppies are going to die because of this and I suppose this could be enhanced later if this is due to a lack of time.

About the "bool exceptions", I would put it as a member variable though, because it changes the behaviour of the class, similarly to the "debug" var, etc.