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

This changes Switch from being a subclass of the Toggleable widget to
being a standalone Component with a custom RenderObject. This is to
enable adding Switch-specific radial reaction animations in a
subsequent patch. The CustomPaint logic that Toggleable was using does
not provide a nice way for a class and its subclass to both participate
in deciding to repaint.

Combine all the scrollable classes into scrollable.dart

We ended up with a bunch of different scrollable widgets and it was confusing.
This CL moves them all into one library to make them easier to find.

Bump versions

Fix Dart analyzer warnings

@abarth

RenderStack should handle top, right, bottom, left all being zero

Sporadic crash from invalid touch event

This required some changes to AnimationPerformance to better understand
animating with forces.

Previously, we didn't set the width of the child correctly when top and bottom
(or left and right) were both specified.

Fixes #275