Freeze installed formulae.

See mxcl/homebrew#18386. Problems and thoughts:
* Side effect: "frozen" becomes a version alias of the frozen version. Is
  this desirable or a Bad Thing?

(As a side note, I feel that `Formula#installed_version` is incredibly broken as it returns a non-existent directory corresponding to the version that would have been installed if the formula was up to date. That is, if I have `git-1.8.2.5` installed, but ran `brew update` and got a `git.rb` corresponding to `git-1.8.2`, `Formula.factory('git').installed_version` returns a `Pathname` pointing to `/usr/local/Cellar/git/1.8.2/`. Is that by design? I've had to work around it using some ugly `Pathname` magic in `formula_freeze.rb`.)