Cleanup base64 code

Use -[NSData base64Encoding] which exists since iOS 4.0 (but is only documented since the iOS 7 SDK).
Delete base64 tests
Fix a bug when creating crash report textual representation

It was working always before, but isn't actually correct.
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
Don't use a category in BITCrashReportTextFormatter

This allows the framework to be used inside another framework.
Hide deprecation warning

Even when using iOS 7 only as base SDK calling `base64Encoding` causes a deprecation warning.