Add pry-stack_explorer support

This is a copy of @jasonLaster' s patch to add stack_explorer support to the original pry-debugger plugin - updated for pry-byebug.  There are cleaner ways to do some of these things in Ruby 2.0 - and this merge is _gross_ but if you want to merge it in so you can mess with it - it works for me.

I'd be happy to give it a second glance if I ever get some time.