Updated graphViz to work on OS X

on Mac OS X the way to open a file with the default handler is "open"

I added a switch to reflect this
Fixed code style
Merge pull request #67 from onigoetz/patch-1

Updated graphViz to work on OS X
Add PR#67 to CHANGELOG
Fixed Image::setFormat() sets Image::type instead of Image::format.
Merge pull request #70 from FGM/image-format

Fixed Image::setFormat() sets Image::type instead of Image::format.