add google spreadsheet support for references

[tsatse]

currently, references can only be included as a raw csv file at the root of the google drive tree.
I've started adding support for google spreadsheet format instead but I've stumbled upon a cross-domain error that seems to originate from a google drive bug (see googleapis/google-api-php-client#102).
my work is in the dev-references-page branch
more details :
here's the documentation from google drive api to download files :
https://developers.google.com/drive/web/manage-downloads

I'm trying to use the exportLinks to get the file in text/csv format but I get the following error :

XMLHttpRequest cannot load https://docs.google.com/spreadsheets/export?id=1zZTmKi4EgM-AYG7XvydMvQd3I-xmC8rlMsjkSvTlDFw&exportFormat=csv. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://drivein.example.com:8000' is therefore not allowed access.

I'm using a simple local web server and I changed my /etc/hosts file so that drivein.example.com points to 127.0.0.1