Added ability to download pgn file for both index and show views

Mark,
I found a small bug in Game#sign, which you should definitely bring into your code.

The other code I wrote allows downloading of a PGN file based on the Games Search mask.
I would still like to find out if Rails 4 allows streaming of output, as this would allow us to generate the PGN file on the fly, without needing to build the whole of the file in memory.