Add support for GET HTTP method when using JSON-RPC service binding

In some specific scenarios, the JSON-RPC allows one to choose between using a GET or POST, particularly when the procedure is safe, idempotent and compatible with HTTP GET semantics. This issue is to add GET support for our json-rpc service binding and tweak cache-control headers to allow some type of cache mechanism to be done when using GET. See [1] and [2] for other references.

[1] http://json-rpc.org/wd/JSON-RPC-1-1-WD-20060807.html#ChoosingBetweenPostGet
[2] http://groups.google.com/group/json-rpc/web/json-rpc-over-http