Attempt at stable packages for wiki

This is a separate request, as I'm not actually sure it's desirable.

### This implements stable-package building for wiki packages. ###
A wiki package's stability is determined by a :stability property.  
If this property is absent (which, of course, is the case for all packages right now), is defaults to *stable*. 

I made *"stable"* the default because packages from the wiki aren't usually considered development versions.

This already includes a previous (separate) commit about checking out tags in git packages.