Added highWaterMark option to cross refference bufferSize

Since node@0.9.12 and on, the `bufferSize` option has been removed and added `highWaterMark`. The default is 16kb but I was seeing your library with node@0.10.13 pull down max of 64kb on my system. 

This change will allow users to define the maximum number of bytes to store in the internal buffer before ceasing to read for node versions 0.9.12+ while still working with node 0.8.x
 
See more: https://github.com/joyent/node/commit/b0f6789a78dbb14778769d1d76b195c2fcd8a340