Add a podspec

Adds a .podspec file, so edn-objc can be used w/ Cocoapods. I'm a complete noob at Objective C, but this Works For Me. I was able to add 

```ruby
pod 'edn-objc', :git => 'https://github.com/arohner/edn-objc.git'
```
to my podfile. I haven't done any work to package & publish an official release on cocoapods.org.