BIG TIME CHANGES.

Using Marshal, instead of YAML. Reduces my library of +28,000 commands
from 168Mb to 2.1Mb. Also reduces the run time from 4.5 minutes to
5secs.

Included a check for whether the existing [old] format is used, and
migrate it to the new version.

persistent-shell-history.gemspec

@@ -16,6 +16,7 @@ See README for other implementation helpers.
 }
 
   s.rubyforge_project = "persistent-shell-history"
+  s.add_dependency("json")
 
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")