Cocoapods and warning fixes

* Add CocoaPods support for both the original repository (https://github.com/blladnar/RBVolumeButtons/pull/10) and this fork. The fork `RBVolumeButtons@rnaharro` podspec could [`pod trunk push`](http://guides.cocoapods.org/making/getting-setup-with-trunk.html#deploying-a-library)ed to the official CocoaPods repository.

* Minor build warning fixes.