Add Code Climate badge to README

Hi,

I noticed that you've got an impressive GPA on Code Climate -- you only have 1 F and 2 Cs! That's impressive for such a widely used open source project.

I work at Code Climate and this PR just adds a dynamic badge displaying the code quality GPA to the README, in case you want this information to be available to contributors.

If you have any questions or feedback, please let me know. I hope you find Code Climate useful.

Cheers,

Noah

PS - Your code metrics are here in case you haven't seen them: https://codeclimate.com/github/binarylogic/authlogic