Fixes issue #169

- Modified HttpClientRequest to take an Observable for content instead of the earlier ContentSource.
- Retained the capability to use an Observable<S> where S != type of the HttpClientRequest. This mode requires passing a function that converts S to ByteBuf.
- Removed the RepeatableRequest functionality as it is not required. Re-subscribing to the content observable should replay the source.
Merge pull request #176 from NiteshKant/master

Fixes issue #169