Added json format to grid#show so can get now and next json feeds.
Added json format to grid#show so can get now and next json feeds.

task 65: Create XL version 
http://github.com/geeksoflondon/grid4rails/issues/issue/65
geeksoflondon/grid4rails,geeksoflondon/grid4rails,geeksoflondon/grid4rails