Support for Zero args and CocoaPods

Hello,

Found your project and loved it's elegance!

This pull requests has the following updates:

- [x] Updated to support zero argument version of `PubSub(subName)`.
- [x] Added an explicit MIT license file
- [x] Created a podspec file

If you like any of these changes and consider merging them in, you'll want to update the `PubSub.podspec` to point to your name/project.

Thoughts?