Add NanoStore.podspec to the project

Hi there. I was wondering why the podspec for CocoaPods wasn't part of this repo. In my opinion it would help getting started with NanoStore (and keeping it up to date). If there is a good reason not to include it, please excuse my PR, I couldn't find a discussion about it in the issue list.

I used @siuying's base version ([2.7.7](https://github.com/CocoaPods/Specs/blob/master/NanoStore/2.7.7/NanoStore.podspec)) for the podspec, only the version number is higher (2.8.0). Thanks for considering.