Fixed TMOAuth crash on iOS 6 and earlier. Fixes tumblr/TMTumblrSDK#56

iOS 7 introduced a new API for base64-encoding, -[NSData base64EncodedStringWithOptions:]. This
API was not available in iOS 6 and earlier, but another API, -[NSData base64Encoding], was. It
was deprecated in iOS 7. So this commit changes TMOAuth to use the new API if available, and
fallback to the deprecated API if it is not available.
Merge pull request #57 from stevestreza/master

Fixed TMOAuth crash on iOS 6 and earlier. Fixes tumblr/TMTumblrSDK#56