Cocoapods

@vikrum Here's a working podspec for geofire-objc.  I modeled it after the podspec in firebase-client-objc.  Does geofire-objc need all the release logic in firebase-client-objc/release.sh?  I figured since this library is open sourced, and much less updated, it wouldn't be too hard to just bump the version number in the podspec and do a `pod trunk` every once in awhile.