idl2go: use generic error instead of *JsonRpcError
idl2go: embed raw JSON in generated .go file
server: add Filter mechanism
server: add Cloneable iface to enable server instance per-request
Add tests for filters and cloneable
- idl2go: convert result to concrete type expected
- convert.go: expose public Convert() function (for use w/idl2go generated code)
- barrister.go: add Idl.Method() to fetch method from IDL. Modify HttpTransport to support http.CookieJar, and a visitor interface for intercepting request headers. Make Transport/Serializer public in RemoteClient. Modify Filter interface to use a RequestResponse struct.  Modify Server methods to accept/passthrough headers.
Update tests to pass in headers map