Clean up HTML

- Rename "import" to "toggle"
- Add/clarify documentation
- Put the JSON data section in a nice box too
Add button to clear the calendar
Improve error message if JSON.parse fails
Improve formatting of generated JSON data

Instead of:

{
    "coordinates": [
        {
            "x": 0,
            "y": 0
        }
    ]
}

generate this:

{
    "coordinates" : [
        { "x": 0, "y": 0 },
        // ...
    ]
}
Add button to generate data from user-entered text
Add self to contributors