It looks quite nice: couple of non-code related suggestions - definitely submit it to cocoacontrols.com where there's a nice growing library of these sort of code snippets, but also maybe think about putting a screen-shot into the readme?
Yes to both please. It frustrates me when people don't put screenshots in their github repos. I tend to skip over them for inclusion in cocoa controls.
See also AQGridView, which is similar in nature but more mature.
Here's another alternative, KKGridView – has support for some pretty cool stuff like sections with headers.