JPEGStream: add progressive mode option

Progressive jpegs have several very useful advantages over baseline
jpegs:

* Slightly smaller for the same quality (3-7%)
* Load much faster in most browsers.

This adds the `options.progressive` option to the
canvas.jpegCreateStream() function.
Merge pull request #309 from iFixit/progressive-jpegs

JPEGStream: add progressive mode option
Readme: update docs for canvas.jpegStream()

Include a note about the lack of an asynchronous implementation.
Merge pull request #310 from iFixit/readme-update-jpegstream-docs

Readme: update docs for canvas.jpegStream()
Initial commit of extracted PNG creation.
Added compression level argument to Canvas::toBuffer
Cleaned up code and added the same fix to Canvas::StreamPngSync
More cleanup.
Added another optional argument for setting row filters.
Merge branch 'master' of github.com:LearnBoost/node-canvas
Release 1.1.0