Adds support for the 'frozen' flag on 'berks upload'

fixes #320
Created a frozen attribute in CachedCookbook (to remove the hardcoded value)

Ran into a few problems:
'thor gem:installed' against master installed correctly, but I was unable to run 'berks upload'

This patch was created of off 1.1.2 (but should merge clean)

Is this a correct approach to fix this?
Is the current master stable (i'm guessing the problem is on my end, but I'll ask before digging in) ?

I'll see about adding a test as well...