Fix order of returned items / updated documentation

[michaelperrin]

This is a pull request to fix the item order problem with Ajax Chosen as mentioned in #32 .

This fixes the problem for both grouped and non-grouped items but it breaks compatibility and it uses an other way to send data to Ajax Chosen: it has to be encapsulated in an object with value and text properties. However, the old format is still supported for backward compatibility purposes (but order is not kept in that case, as before).

The updated documentation (README.md file) is enclosed in this pull request and can be reviewed as well.

UPDATE: backward compatibility is now kept

[michaelperrin]

I now made some changes so that backward compatibility is kept, which is much better!

You can now pass data like this (old format):

{"3": "Ohio"}

or this (new format):

[{"value": 3, "text": "Ohio"}]

For groups, the data can look like this (old format):

{"3": {
    group: true,
    text: "City",
    items: {
        "10": "Stockholm",
        "23": "Moskow"
    }
}

or this (new format):

[{
    group: true,
    text: "Europe",
    items: [
        { "value": "10", "text": "Stockholm" },
        { "value": "23", "text": "Moskow" }
    ]
}]

I kept the documentation with only the new format as there is no problem with order. This is therefore the recommended one.