Add Code Climate badge to README

Hello,
 
Just wanted to drop you a note to let you know that Whenever has been added to Code Climate. Code Climate is a service I built that provides code quality metrics for Ruby apps and libraries, and is free for OSS. You can view the data for Whenever here:
 
https://codeclimate.com/github/javan/whenever
 
This is a small pull request that adds a Code Climate badge to the README, in case you want this information to be available to contributors, like RSpec does:
 
https://github.com/rspec/rspec-rails#readme
 
Let me know if you have any questions! Hope you find it useful.
 
Cheers,
 
-Bryan