Add timespan to gantt. Add to demo.

In my app, I want to give more context to the tasks. To be specific, I want to show a timespan they selected in a different part of my app.

To do this, I added a new feature called "Timespans". We can load timespans into the gantt view, which have a data structure like tasks, then see a highlighted box running from the top to the bottom of the Gantt view behind the tasks.

Timespan objects have a `classes` property, so we can control the styling of each timespan. I wanted to add opacity, borders, and background color.

See a demo here: https://rawgithub.com/chexxor/angular-gantt/feature/add-highlighted-timespans/demo.html

I took a bit of time cleaning this up to share. Thanks for sharing this nice module, Schweigi!