[ENHANCEMENTS THAT MAY BREAK YOUR CODE]

• The $mech->stack_depth() setting had no way to say "don't cache any
  pages at all". How silly!
  

Now, if you set $mech->stack_depth(0), no history of pages will be kept.
In the past, it would mean "Keep all pages." This means that if you want
to set it to keep all pages, set it to some ridiculously large number.