Change task.color to task.classes.

I made a better fix for the problem this issue - https://github.com/Schweigi/angular-gantt/pull/32

If we change the `task.color` attribute to `task.classes`, we can add any number of classes to each task, which defines text color, background color, or even border styles. I placed these task-specific styles in a separate CSS file, called "gantt-user.css".

You can test it here: https://rawgithub.com/chexxor/angular-gantt/feature/add-task-classes/demo.html

Thoughts? Also, is the "develop" branch the right target for this patch?