Add pubspec.yaml files for each of the examples/ directories

so that pub get will create a packages/ directory for each
and users of packages/sky/sky_tool can run these examples
from sky_sdk as the instructions say to.

TBR=ianh@google.com
https://github.com/domokit/mojo/issues/128

Review URL: https://codereview.chromium.org/1110283002

examples/README.md

 Sky Examples
 ============
 
-This directory contains several examples of using Sky.
+This directory contains several examples of using Sky.  Each of these is an
+individual Dart application package.  If you wish to run them with `sky_tool`
+then you will want to run `pub get` inside their directory before running
+`./packages/sky/sky_tool start`.
 
 1. *Hello, world.* The [hello world app](hello_world) is a basic app that shows
    the text "hello, world."

examples/color/pubspec.yaml

+name: color
+dependencies:
+ sky: any

examples/hello_world/pubspec.yaml

+name: hello_world
+dependencies:
+ sky: any

examples/raw/pubspec.yaml

+name: raw
+dependencies:
+ sky: any

examples/widgets/pubspec.yaml

+name: widgets
+dependencies:
+ sky: any