Add info about pub variables and carrot syntax.

Cleaning up the installing page.

BUG=https://github.com/dart-lang/www.dartlang.org/issues/927
BUG=https://github.com/dart-lang/www.dartlang.org/issues/1203
R=nweiz@google.com

Review URL: https://codereview.appspot.com//185610043