adds verbose error reporting when playback fails to find file

example:
irb(main):001:0> playback("nothing")
Screenshot at screenshot_0.png
RuntimeError: Playback file not found for: 'nothing' =>
  searched for =>
    * 'nothing_ios6_iphone.base64'
    * 'nothing_ios5_iphone.base64'
  in directories =>
    * '/Users/moody/git/xamarin/juke/LessPainfull/playback'
    * '/Users/moody/git/xamarin/juke/LessPainfull'
    * '/Users/moody/git/xamarin/juke/LessPainfull/features'
    * '/Users/moody/.rbenv/versions/1.9.3-p392/<snip>/lib/calabash-cucumber/resources/'
Merge branch '0.9.x' into 0.9.x-playback-verbose-reporting

Conflicts:
	calabash-cucumber/lib/calabash-cucumber/core.rb
added comment for the interpolate function - never called?
made the playback search algorithm easier to understand
Merge pull request #181 from jmoody/0.9.x-playback-verbose-reporting

adds verbose error reporting when playback fails to find file
Merge branch '0.9.x' into 0.9.x-playbacks-should-not-be-saved-to-local-dir

Conflicts:
	calabash-cucumber/lib/calabash-cucumber/core.rb
now does exhaustive searching for compatible playbacks, including when
device is ipad.

replaces the Dir.exists? call with File.directory? to be compatible
with ruby 1.8.7