onCellEnter & onClose callbacks added

added two optional callbacks that can be useful for devs

- onCellEnter - for when the user mouses over a color cell. useful if you need to tell other parts of the application when a color is being moused over (previews etc).

- onClose - executes whenever the color selector is closed for any reason (selection, blurring).

Also included is a more discerning bind logic that only listens for clicks (to close) when the color selector is open and is unbound when it's closed:

``` javascript
$('html').bind("click.simpleColorDisplay", function() {
  $('html').unbind("click.simpleColorDisplay");
  $('.simpleColorChooser').hide();

  if (options.onClose) {
    options.onClose();
  }
});
```

usage
```javascript
$('.colorselector').simpleColor({
      onCellEnter: function( hex ) {
        console.log("You just entered #" + hex);
      },
      onClose: function() {
        alert("color selector closed");
      },
  });
```