Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

As part of this:
 - A lot of classes got new lerp functions, including e.g. TextStyle.
 - Theme's constructor story got overhauled. You can now configure
   everything if you really want to, and we're better about defaults.
 - Material no longer automatically animates its background color.
   (It still does for its shadow.)
 - Tabs try to get the indicator color from the theme.
 - The fields in ThemeData got reordered for sanity.
 - Theme.== and Theme.hashCode got fixed.
 - Typography got a bit of a spring cleaning.

Fixes #613.

A MaterialList understands the sizing, padding, and scrollbar features of
Material Design lists.

Also, add CircleAvatar for showing the circular avatars that are commonly used
in material lists.