Handle batch object cookie read/write operations

This implements, tests and documents $.cookie() batch read/write operations:

    $.cookie() // => { all: "cookie", values: "present" } – fixes #16

    $.cookie({ all: null, values: "tweaked" }) // deletes "all" and edits "values"

    $.cookie(obj, options); // batch op with a common options arg used for all

I also fixed the test suite to work right, even if run on a server you have other cookies on.