Get data from Spreadsheets as JSON so a CORS proxy is not needed

JSON requests to the Spreadsheets API support CORS. Can't really find documentation for this, but it works. Only potential issue is support for IE8/9 if hosted on non-HTTPS, as Google requires HTTPS (see item 7 here: http://blogs.msdn.com/b/ieinternals/archive/2010/05/13/xdomainrequest-restrictions-limitations-and-workarounds.aspx)
Merge pull request #1 from SimonEngelhardt/master

Get data from Spreadsheets as JSON so a CORS proxy is not needed