Replace StreamBuffer with CircularBuffer
Complete rewrite!!

- The code is simpler and easier to read
- You can now give standard input to `run(...).exec([stdin], [callback])`
    as a String, Buffer, or Vinyl file
- A new `Command` class is exposed for running commands outside of gulp
- Fixes #8
- No need to set the max listeners on `process.stdout` (#7)
- Removed dependency on CircularBuffer