Update podspec for OS X SDK
Remove NSData+(Jive)Base64

NSData(NSDataBase64Encoding) already provides base64 encoding
Remove NSData+JiveBase64 from podspec
Prevent deletion of jive-ios-sdk-tests/lib

As the directory is now empty, `git clean` would delete it.
We don't want it to go away as it's used for a group in xcodeproj.
Merge branch 'feature/osx-sdk' into develop
Merge branch 'feature/using-system-base64_encoding' into develop