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
Revert "Support Swift language changes in Xcode 6.1", fixes #39

This reverts commit bab23f6d3efc3b0a0c89a354ec344827c2288c82.