Additional features 

Hi Marco,

Apologies if I'm doing this wrong - although I've used git for a while privately I've never tried to contribute to open source before.

I have created two new features which you may wish to pull in to the main repo:

1) When using the day column view, it is now possible to show/hide non-work hours.  This means that, although the column looks the same as a normal day column, the time range it displays is just the working hours.  Any times outside of the working hours are just positioned at the far left or far right of the column.  I have updated the demo files to make it easy to see how this behaves.  As I describe in the commit message, dragging and dropping goes a bit weird if the number of working hours displayed is not divisible by the sub-scale.

2) When a period of time is hidden (for example when weekends are not showing, or when non-working hours are hidden) then the vertical line on the gantt between two non-contiguous time blocks represents two different times.  This causes undesirable behaviour when moving or resizing tasks with the drag and drop functionality (which is awesome by the way).  I have fixed this so the date that the task snaps to depends on whether it is the start or the end of the task.  My commit message goes in to more detail.

Let me know what you think!

Andrew