Fix property name for entry object

It appears from a raw log of entry object that the property name is 'gs:cell'
Fix usage of saved reference to 'this'
Check dynamically response format from WS calls

Google Spreadsheet services may return an empty object if when asked for JSON format (alt=json in url)
Try to parse JSON output in request method, and then try to parse it as XML if the JSON attempt failed
Should solve #29
Merge pull request #33 from Pingflow/pr-response-format

Pr response format