Sync and Async XmlHttpRequest client modules for javascript.

In general async is the right thing.

To work in the iocamljs top level synchronous is much more convenient, so it's provided here as well.

Note; removed the Request/Response modules which were external to the Client.  Not sure why they were needed.
If we have one recommended one (in this case, Client_async), could we just call that module `Client` instead?  Simplifies the surface API.