Add directives that can be used to control scrolling

[joelmukuthu]

Add these directives:

• scroll-up
• scroll-down
• scroll-top: a two-way bind to the scrollTop of the element
• scroll-animation: with values: true, false or "animationDuration<number> animationEasing<string>" e.g. "400 swing". true would cause it to use the defaults: defaultSnapscrollSnapDuration and defaultSnapscrollScrollEasing

This would require some refactoring in the code to expose code that is in the snapscroll directive as services that can be shared by these directives.

[joelmukuthu]

Scroll-related functionality has since been pulled out into a separate module https://github.com/joelmukuthu/angular-scrollie. These directives would apply there.