Add support for time_scale (Range) option

![screen shot 2013-05-02 at 3 03 33 pm](https://f.cloud.github.com/assets/99602/456731/2ba78022-b375-11e2-8850-8387d54b41e6.png)
First off, thank you for the awesome work!

I added support for an optional time scale range, so the calendar adds a first column including a "time scale" for scheduling stuff.

Usage:
```ruby
weekly_calendar time_scale: (8..17)
```

I requires two more methods for the Event class in order to add a couple of attributes which can be used with CSS to set the height and position of the event: `duration` (minutes) and `start_time_offset` (offset in minutes from the start of the scale).

Let me know your feedback and don't feel obligated to merge this into master, I just thought someone else could benefit from this feature.