Cleanup base64 code

Use -[NSData base64Encoding] which exists since iOS 4.0 (but is only documented since the iOS 7 SDK).
Delete base64 tests
Merge branch 'base64' of github.com:0xced/HockeySDK-iOS into 0xced-base64 with modifications

- Don't use `base64Encoding` if the SDK is build with iOS 7 as based SDK, since that would cause a deprecation warning