Modules: added Request, Response and Headers ctors in Fetch API.

Modules: added Request, Response and Headers ctors in Fetch API.

    Added Headers method and properties: append(), delete(), get(),
        forEach(), has(), set().
    Added Request method and properties: arrayBuffer(), bodyUsed,
        cache, credentials, json(), method, mode, text(), url.
    Added Headers, Request, Response constructors.

    This closes #425 issue on Github.