(feat) custom eventRender support

You can now render events in a custom manner via fullcalendar's `eventRender` option.
However, only certain event attributes are watched for changes by default.
They are: id, title, url, start, end, allDay, and className.

To watch additional attributes pass an expression that returns a string to `calendar-watch-event`.
The expression is evaluated with `event` local variable, e.g.:

    <div calendar-watch-event="event.price" ui-calendar="calendarOptions" ng-model="eventSources">
    or:
    <div calendar-watch-event="someScopeMethod(event)" ui-calendar="calendarOptions" ng-model="eventSources">

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did not push straight to master, because no tests (still haven't learned how to write those)