Refactors matrix handling in sprites

This file was deleted.

Add a widgets.dart uber library

Now you can import 'package:sky/widgets.dart' to get all the widgets.

* Intentions => Behaviors
* SlideIn => SlideTransition
* FadeIn => FadeTransition
* removed unused AnimatedContainer intentions

Use transitions for Dismissable

Fix visual bugs in Stocks

Simplifies the code a bit.

 - Center the labels on the tabs
 - Make the drawer's mask visible

When a ScrollableBlock can scroll, we want to be able to drag into the
overscroll region. Previously we could fling into the overscroll region, but we
couldn't actually drag there.

Support borders with borderRadius

Use those in SnackBar, Drawer, navigator instead of AnimatedContainer's
intentions.

Currently we support only borders with uniform color and width.

Use absolute URLs in sky/packages/sky/README.md

When this README.md is published to pub.dartlang.org, the base URL is
pub.dartlang.org. We need to use absolute URLs in order to get back to GitHub.

Fixes #417

Fix TextAlign.right

We were applying the style to the RenderInline but we actually needed to apply
it to the RenderParagraph. The lineHeight property had the same problem.

Stacked does not repaint when its position changes

We were marking the parent *Widget's* render object as needing layout, but
we actually need to walk up the component tree until we find the parent
*RenderObjectWrapper* so we can mark actually mark the parent render object as
needing layout.

Fixes #408

Fixes anchor points for particles in particle systems

Make Switch a standalone class with a custom RenderObject