Implement placeholder field property

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.