Create Podspec

Allows to instal GrowthPush from [CocoaPods](http://cocoapods.org).

Also avoids conflicts with projects that already use/use a different version of AFNetworking.
Add missing header contents
Merge remote-tracking branch 'fork/cocoapods'
Update podspec for version 1.0.6

Requires a 1.0.6 tag.
Merge pull request #1 from rivera-ernesto/cocoapods

Create CocoaPods' Podspec