Add support for Cross Origin Resource Sharing

[vrutsky]

On order to use jsonrpc library as server in crossdomain communications with JavaScript client in web-page, server should implement CORS [1].

Not sure how correctly implement it, but I managed to use it with following workaround outside jsonrpc library --- handle OPTIONS request that is send by browser to check whether CORS is allowed and add allow headers in all responses:

def add_CORS_headers(request):
    headers = request.responseHeaders
    headers.addRawHeader('Access-Control-Allow-Origin', '*')
    headers.addRawHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS')
    # This is necessary for Firefox
    headers.addRawHeader('Access-Control-Allow-Headers', 'Origin, Content-Type, Cache-Control')


class MyServer(JSON_RPC):
    def render_OPTIONS(self, request):
        add_CORS_headers(request)
        return ""

    def render(self, request):
        if request.method == "OPTIONS":
            return self.render_OPTIONS(request)
        else:
            add_CORS_headers(request)
            return JSON_RPC.render(self, request)

[1] https://secure.wikimedia.org/wikipedia/en/wiki/Cross-origin_resource_sharing