added support for object parameters

[pilot4u]

added costume serializer adn deserializer to support JSON objects as parameters

[pilot4u]

Sample usage for those capabilities :
createBrowserApp(AppNavigator, { history: "hash", paramsSerializer: p => "params=" + JSON.stringify(p), paramsDeSerializer: p => JSON.parse(p.replace("?params=", "") || "{}") })

[infostreams]

Can this be merged please? @ericvicenti

EDIT: Just so that I can do

        createBrowserApp(SwitchNavigator, {
            paramsSerializer: params => {
                Object.keys(params).forEach(key => {
                    if ((typeof params[key] === 'function') || (typeof params[key] === 'object'))
                        delete params[key]
                })
                return queryString.stringify(params)
            },
            paramsDeSerializer: p => queryString.parse(p)
        })