This commit fixes a bug in form dialogues where the checkbox property is not actually toggled back a second time when using the label text. The issue is caused by the use of the `checked` attribute (`.attr('checked'`) because changing this attribute is not guarenteed to update the element `checked` property. Attached is an image proving how this can arise in an updated version of Chrome. See how the attribute is present but the property does not match. This was after clicking the `span` element a second time.

Using the `.prop` method of the jQuery we use fixes this issue totally. It is suggested other usage of `attr` on checkboxes is revised.

Proof: http://i.imgur.com/KGM9LUd.png

Remove extra line space

Checkbox bug fix

Now the new complete file can be built

TODO: Minify and update documentation.

Added field placeholder property,

eg

```javascript
username: {
title: 'Username',
width: '20%',
create: true,
placeholder: "Optional",
key: true,
sorting: false
},

```
Tested safe back to IE6 (placeholder attribute not supported but is
simply ignored). Works on IE10+. Also tested on Chrome and several
mobile browsers.