Rewrite!

I did a complete rewrite. The code is much clearer now, and there are a few new features.

• run(...).exec([stdin], [callback]) can now take standard input for the command.
• A new run.Command constructor is exposed for running commands outside of gulp pipelines.
• Users no longer need to change the max listeners on process.stdout (see #7).
• Gulp-run no longer mangles the names of files in a pipeline (fixing #8).
• Gulp-run no longer depends on CircularBuffer, instead it uses the core stream.PassThrough.