factor.mix does not have a describe method

I'm new to Zelig. The lack of a describe method appears to prevent any use of this function, but I'd be happy to learn otherwise.

```
Warning messages:
1: In zelig(cbind(TakenPDC, YrsInv, PresInvolv, FutInvolv, SocPerc) ~  :
  invalid object returned from `zelig2` method
2: In describe.default(object) :
  The 'describe' method for this function is unspecified
```
