Add Code Climate badge to README
extract Pry::RescuableException to lib/pry/rescuable_exception.rb
pry.rb: Move all exception specific code to its own file

This finishes up Robert Gleeson's refactoring. A sole file for
RescuableException doesn't make much sense, since we have a few more
exceptions left behind.
Revert "Build this branch on Travis [temp]"

This reverts commit 2cc60c704d4ca709c9b10b7556a2725c2a90f167.

Please, use forks in order to test your code on Travis, because
currently it borks our Travis. Thanks! :-D