use classForCoder to get valueClassString if possible

fixes https://github.com/joeldev/ADNKit/issues/63
Merge pull request #79 from rrbrambley/master

use classForCoder to get valueClassString if possible
Merge pull request #76 from berg/master

Upstreaming a bunch of stuff we use in the App.net app
Merge pull request #80 from berg/master

Fix a few readonly-property bugs that I introduced.