Allow `init-date` to be set just as `min-date`

Currently, we can bind `init-date` attribute only to `datepicker` attribute.

If I set with on element with `datepicker-popup` directive it is ignored. Example:
```html
<input type="text" datepicker-popup="true" init-date="2014-02-18" />
```

It isn't stored also in global `datepickerConfig` - why not, if `min-date` and `max-date` is there at the same time?