Moved to readable-stream. Adds support for node@0.8

I would love to use this module but it doesn't currently support `node@0.8` which I am currently a proponent of until `node@0.12` comes out. To fix that, I have moved over to `readable-stream` which is the user-land version of `stream` in `node@0.10`. In this PR:

- Added `node@0.8` testing into Travis CI
- Downgraded `node` engine to `node>=0.8`
- Added dependency to `readable-stream`
- Moved from `core's stream` to `readable-stream`

Relevant links:

https://github.com/isaacs/readable-stream

http://r.va.gg/2014/06/why-i-dont-use-nodes-core-stream-module.html