fix "Generator is already running" errors when no callback provided

this is https://github.com/visionmedia/co/pull/50, but specifically when no callback is provided. adapted your diff and only the relevant test cases are included.

note: this removes arguments support because it becomes too complicated to check if the last argument was a callback. i'm going to do a PR for https://github.com/visionmedia/co/pull/50#issuecomment-28863545 anyways.