* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

Fixes #1382

Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.

This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.

This object represents a rect the auto-layout system.

This patch makes it easier to use the auto layout API:

* We no longer use operator== because that requires an ugly cast by the
  API user.
* Also, "leftEdge" is now just "left" for less verbosity.
* AutoLayoutChild not implies its key from the AutoLayoutParam object.
* We now correctly layout every child of a RenderAutoLayout object even
  if the solver doesn't flush any updates to that child.

This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.

Previously, hello_world.dart was an interactive circle. I've moved that
to touch_input.dart. We should eventually harmonize the touch input
examples at all the layers.

Our examples have been growing organically over time. This patch cleans
them up to illustrate specific aspects of Flutter.