(feat) custom eventRender support

You can use fullcalendar's `eventRender` option to customize how events are rendered in the calendar.
However, only certain event properties are watched for changes (they are `id`, `title`, `url`, `start`, `end`, `allDay`, and `className`).

If you need to re-render other event data automatically, with this PR you can use `calendar-watch-event`.
`calendar-watch-event` expression is evaluated with an `event` and should return a string or a number, for example:

    <ui-calendar calendar-watch-event="event.price" ... >
    <ui-calendar calendar-watch-event="someScopeMethod(event)" ... >