define node version supported and dep on readable stream.
make use of readable-stream.
add readable-stream fallback for readable and writable.
borrow clamp function from node since buffer.slice in v0.8 doesn't have such logic.
quick tweaks to clamp.
Merge pull request #6 from ctalkington/readable-stream

use readable-stream for initial v0.8 support.
v0.2.4