Add preprocess to support unix and utc

I added configuration `preprocess` to support `moment.unix`, `moment.utc`..etc.
You also could overwrite the configuration in the directive `am-time-ago` with `am-preprocess`.
```js
app.constant('angularMomentCofnig', {
  preprocess: 'unix'
});
```
`am-time-ago` would `preprocess` with `moment.utc`.
```html
<span am-time-ago="model.created_at" am-preprocess="utc"></span>
```