Adds pause() and resume() methods to the S3 upload stream.

Calling pause() will:
* emit a 'pausing' event with the number of mid-upload parts.
* immediately halt stream input, until an internal 'resume' event.
* delay queued up (but not yet sent) parts until the 'resume' event.

Calling resume() will:
* fire the internal 'resume' event
* emit an external 'resume' event

When all mid-upload parts complete, a 'paused' event fires with the
current UploadId and Parts data, suitable for freezing for manual
resumption at a later time.

pause() on a dead or paused stream does nothing (returns false).
resume() on an unpaused stream does nothing (returns false).
also return true on resume under normal circumstances