Added podspec file for Cocoapods integration

Hey there --

I'm looking to use your library in a project where I'm using Cocoapods to manage my dependencies, so I thought I'd try to create a pod for the project.  This is my first time making a pod, so I'm just following the directions at http://cocoapods.org/ and the docs at http://docs.cocoapods.org/specification.html.

So far as I can tell, there are only two things that have to be done to support Cocoapods integration:

1) The `s.prefix_header_contents` section in the .podspec file must always reflect the contents of your `Balanced-Prefix.pch` file.
2) You must tag your releases with a version number and update the `s.source` line in the .podspec file to reflect the current tagged version.  (Cocoapods requires the tagging to distribute the proper commit.)

If you accept this pull request or create a podspec file of your own, I'd be happy to submit the ticket to Cocoapods to get them to start distributing the package.  Should make integration lots easier!