Add an http_throttle package.

BUG=20058
R=alanknight@google.com

Review URL: https://codereview.chromium.org//418103004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/http_throttle@38560 260f80e4-7a28-3924-810f-c04153c831b5
Add gitignore, status, and codereview files.
Update the pubspec's homepage link.