Support Swift language changes in Xcode 6.1

(Note these changes will not work with Xcode 6.0)

- Added UnicodeScalarLiteralConvertible protocol to JSON
- Added explicit unwrapping where needed e.g. optional NSURL (ideally
we should check for errors first before using the returned objects),
UITableViewCell constructor is now also optional
Merge pull request #38 from heidiproske/master

Support Swift language changes in Xcode 6.1