[OS X] Add codesign_frameworks to configuration. Use it to specify which frameworks to codesign

Use it like this:

    app.codesign_frameworks << 'Sparkle.framework'

Relevant:

* [TN2206 OS X Code Signing In Depth – Signing Frameworks](https://developer.apple.com/library/mac/technotes/tn2206/_index.html#//apple_ref/doc/uid/DTS40007919-CH1-TNTAG13)
* http://furbo.org/2013/10/17/code-signing-and-mavericks/