feat($dragImage): service for simplifying dragImage generation

Added the `$dragImage` service, which allows much simpler generation of the `dragImage` object. If the interpolated value of the `drag-image` attribute  (introduced in #23) is a string, it is now converted to an image automatically. The generated image can be completely customized by overriding `$dragImageConfig` properties.
You can override the default config for the entire app like this:

```js
angular.module('app').config([
  '$dragImageConfig',
  function (config) {
    config.xOffset = -15;
    config.fontColor = 'black';
  }
])
```

In addition, I just pushed another commit that allows you to override any of the defaults at runtime:

```js
$dragImage.generate('Hello world', {
  xOffset: 0,
  font: 'normal 20px monospace'
})
```